Grinding noise at idle

hello all...I have a problem has me stumped. I've got a well used 305 in my truck that is making a grinding noise at idle only, putting it in gear makes the noise worse as it lowers the RPM's, turning on the AC makes it even worse. With any these conditions once the RPM's crest roughly 1200 the noise completely goes away. I've tried using a homemade stethoscope to pinpoint the noise, but i can hear from the front of the oil pan to the transmission case behind the TC. The only other thing I have noticed is going down the road if I give it more gas to accelerate the oil pressure will drop and when I let off the gas the OP will jump back up 5-10 PSI. Thanks for any help ya'll can give me. :confused:

An easy test is to remove all the belts and start it up. If the noise is internal to the engine, it will still be there. If the noise has gone away, test the external devices one at a time.

As odd as it may sound, you also may want to smell the transmission fluid. If it smells "burned" you could have a transmission or convertor problem causing the noise. The convertor and front pump are moving even with it in Park.

converter was my fist thought

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Originally Posted by firebird77clone

converter was my fist thought

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Or maybe a sloppy timing chain.. Especially in a 305

all great idea's...thanks, i'll post back when i've got more info

Check for crankshaft end play.
It could explain the oil pressure problem.
If the thrust is gone because of the previous set up, it could be wiped out and deleting pressure and causing the throws/counterweights of the crankshaft to swing and hit the block simulating a "grinding" noise.

A bad rear main bearing will cause the oil pressure drop upon acceleration you describe. Could also be the source of the grinding noise at idle. Good luck!

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Originally Posted by nitrowarrior

Check for crankshaft end play.
It could explain the oil pressure problem.
If the thrust is gone because of the previous set up, it could be wiped out and deleting pressure and causing the throws/counterweights of the crankshaft to swing and hit the block simulating a "grinding" noise.

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excellent idea.....so far my paln of attack is to pull all the belts off and see if it goes away, it that doesn't work then i'm gonna unbolt the TC, try to isolate it in either motor or tranny and work from there. Thanks again!!:D

Maybe two separate problems? The grinding could be a cracked flex plate. The oil pressure drop and rise is a plain indicator of loose main bearings. It is conceivable that loose mains could cause vibrations that could crack the plate.

You didn't say what year truck. But if it has the one piece aluminum alternator,power steering,& A/C mount I would look there first,they where prone to cracking and would make a heck of a noise especially when you turned the A/C on.

it's an 82, non serpentine.....I'm thinking, flexplate/TC/trans front pump/crank-bearings.....the diagnosis starts tomorrow, thanks again for the help