What you most likely have is another circuit altogether , controlled by a relay, that has somehow moved on it's base and striking metal..(making it audible)
Something like a Fuel pump relay that goes on/off in spurts to maintain fuel pressure or Some other Data reporting relay making an audible noise..
Do an audio check (stick your head up under the dash..
) and track the noise down , use your finger tips when you get close, you'll probably find a relay on a harness end, adrift and laying on a metal panel amplifying the noise..
I agree..Don't pay for something you can provide a "First line" of defense for! In this situation (Since everything else runs Normal) they are not going to do anything different..except charge you $99.00 to $120.00 an hour to do it..It'd Be different were it a Computer malady, where a terminal would be required.