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Old 09-12-2010, 10:38 PM
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unknown knocking sound =(

my small block chevy started making a funny sound the other day, i could just barely hear it when driving normal, almost sounded like a header gasket leaking (that's how little the difference is) So i decided to run it up and down in rpm as i drove, and i found a spot, at around 4000 rpm, where if i hold the gas pedal steady, or back off a bit it makes a clattering sound, but if i give it more gas it goes away, or if i back off the gas altogether it goes away. It will NOT make the noise in neutral just by revving it, everything sounds perfectly normal in neutral. i've always noticed a bit of a lifter tick at idle, so i decided to pull my valve covers to adjust rockers and check my pushrods (since my lifters are new), and i found that one pushrod was slightly bent, others were off by just a little bit (i held them to a machined straight edge, none were bent to the naked eye, the one you could kind of tell by looking, but only that one)

could this have been what was causing my noise, or do you guys think i have something down deeper, like i said, only at 4000rpm in gear, holding steady or slightly decelerating, never in neutral, never if i accelerate. any thoughts would be greatly appreciated

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Old 09-13-2010, 05:29 PM
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Hard to say exactly what the problem is but if your push rods weren't new they may have been bent a long time ago. There is a possibility that your push rods are to long. Need more info on this such as what type rocker arms you have and the specs on your cam. It could be that your push rods are a little to long and the slot in the push rod is hitting the stud. It could also mean that the came has more lift then the springs are able to handle. If the springs are binding that too could have bent the push rods. Give us a run down on what components you have from the cam to the valves.
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Old 09-13-2010, 09:16 PM
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edelbrock performer rpm cam, Summit Racing SUM-152123 heads, with the springs they come with. I had no previous noises. i just bought stock length pushrods, the ones i had before were stock length, and there seemed to be no clearance problems, the rocker stayed on the tip of the valve. the noise is new, it wasn't there before. and i had previously checked the pushrods with a straight edge and they were straight when i put them in
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Old 09-14-2010, 07:52 PM
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Quote:
Originally Posted by recnepswilliams
my small block chevy started making a funny sound the other day, i could just barely hear it when driving normal, almost sounded like a header gasket leaking (that's how little the difference is) So i decided to run it up and down in rpm as i drove, and i found a spot, at around 4000 rpm, where if i hold the gas pedal steady, or back off a bit it makes a clattering sound, but if i give it more gas it goes away, or if i back off the gas altogether it goes away. It will NOT make the noise in neutral just by revving it, everything sounds perfectly normal in neutral. i've always noticed a bit of a lifter tick at idle, so i decided to pull my valve covers to adjust rockers and check my pushrods (since my lifters are new), and i found that one pushrod was slightly bent, others were off by just a little bit (i held them to a machined straight edge, none were bent to the naked eye, the one you could kind of tell by looking, but only that one)

could this have been what was causing my noise, or do you guys think i have something down deeper, like i said, only at 4000rpm in gear, holding steady or slightly decelerating, never in neutral, never if i accelerate. any thoughts would be greatly appreciated
Double or single knock? More up to about 1500 RPM then either goes away or drowned out by other noise? Relates to engine speed or road speed?

Double knocks are rod bearings, wrist pins, cracks in piston skirts.

Single heavy knock is main bearings, sometimes flywheel/clutch or flexplate/torque-converter on transmission input shaft bearings.

Noise strongest under 1500 RPM tends to be various causes from the valve train.

Noise with engine RPMs and car not moving is usually engine or auxiliary related.

Thuds timed with on and off throttle, especially when moving, tend to mounts or their attachments.

Noises that increase with throttle and decrease with less while car is being driven tend to be u-joints, transmission internal bearings, or rear end. If you have drum brakes on the rear, if a slight application makes the sound go away it's probably outer wheel bearings.

Bogie
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Old 09-14-2010, 10:37 PM
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well, i replaced my pushrods and the knock went away. but the knock was only at 4000 rpm, and sounded like it was moving way slower than 4000 times per minute i guess i got lucky this time and it was just pushrods (i had a pontiac 400 that made the same exact sound at 2000 rpm and it was lifters in that, took it apart and all bearings were fine.)
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Old 09-15-2010, 09:08 AM
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Quote:
Originally Posted by recnepswilliams
well, i replaced my pushrods and the knock went away. but the knock was only at 4000 rpm, and sounded like it was moving way slower than 4000 times per minute i guess i got lucky this time and it was just pushrods (i had a pontiac 400 that made the same exact sound at 2000 rpm and it was lifters in that, took it apart and all bearings were fine.)
The easiest and arguably best way to check a p-rod for straightness, is to roll it across a glass surface. If it is bent, you will be able to see the uneven rotation and you will easily hear it 'tick-tick-tick' every rotation as it rolls across the glass.
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Old 09-15-2010, 11:25 PM
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i didn't have a glass surface to roll them on. i just put them on a machined straight edge and held them up to the light and noticed that on 9 of the pushrods they were flat with the straight edge along the ends, but there was light in the middle, and when i rotated it changed to light through the ends but not the middle. they really didn't look very bent. i was really worried i had a connecting rod knocking. but i guess it just takes a tiny tiny bit. the worst one had maybe a little more than .010 gap between the pushrod and the straight edge.
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Old 09-21-2010, 09:22 PM
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Quote:
Originally Posted by recnepswilliams
well, i replaced my pushrods and the knock went away. but the knock was only at 4000 rpm, and sounded like it was moving way slower than 4000 times per minute i guess i got lucky this time and it was just pushrods (i had a pontiac 400 that made the same exact sound at 2000 rpm and it was lifters in that, took it apart and all bearings were fine.)
At 4000 RPM the cam shaft is only turning 2000 thus any valve train noise sounds like it is not with the engine speed. Lets hope that you have fixed the problem. Bent push rods can make a knocking/taping sound. I would still check and make sure that the rockers are not touching/binding on the studs and make sure that the springs aren't binding.
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