I strongly suggest that you do SOMETHING. The rattling you hear is detonation and will ultimately destroy the pistons, followed shortly by everything else.
I suspect the rebuilder pistons that were installed when you had the block bored are the culprits of the whole mess. Oversize pistons are manufactured with a reduced compression height (distance from the centerline of the wrist pin to the crown of the piston) to compensate for the machine shop cutting the deck of the block. If the deck is not cut (suspecting yours wasn't), then the pistons are too far down in the hole at TDC to make an effective squish (or quench) area between the piston crown and the underside of the cylinder head. If the squish were like it is supposed to be (0.035" to 0.040"), this motor wouldn't be rattling at 150 psi, or 160 or 170.
The reason it rattles more at low rpm's is that the pistons/valves are moving too slow to cause turbulence of the mixture, which helps to prevent detonation. At higher r's, the mixture is more turbulent and you don't hear detonation as much.