A while back, my motor started making a rattling sound when idling in drive. Its not a constant sound, ( as in not even, and changes pitch) and its RPM specific. id say around 500-600ish (no tac in my car) It sounds like its a line touching the frame, or a loose bolt, or nut. I've checked high and low for loose parts or bolts and the rattle remains. i disconnected the PCV valve to see if it was the source of the noise, but it was not. I also checked all the brake, trans, fuel, and metal vacuum lines to see if they were touching anything and they weren't. Then i went after the motor it self with a long screw driver up to my ear, and it seems like the noise comes from the front of the motor. I realize that this noise could be reverberating from another location, but its got me curious about my timing chain. This is a replacement motor i bought from an older gent who was a retired engine builder. It had 60ish thousand miles on it when i bought it, and about 80,000 miles now. As far as i was able to tell at the time, since it had the nylon timing gear, it was the original set. I wish i had changed the set when i installed the motor, but coulda shoulda woulda i guess.... I had to re set the timing recently, as it was suddenly too retarded, and its been so long since i set it originally it was hard for me to tell if the distributer may have moved a bit and that was why. My only issue with that is that its had this rattle for quite a while before i had to advance the timing. What would it sound like if the chain was stretched and slapping the timing cover? My issue with it being the chain is that it runs smooth as glass after i re-adjusted the timing. Could that be my problem? I know its around the right miles for a new timing set. should i just do it and then see if it still rattles?
Thanks guys!

 

You can determine how much slack is in the timing set (chain) by turning the crankshaft with a wrench by hand & monitoring rotation of the distributor rotor when #1 cylinder is on the compression stroke at TDC. Roughly - a reading <5° is acceptable, 7° is marginal & often >7° means time for a new timing set (chain).

Many videos describing this technique. Here is a basic demonstration video.

 

Thanks norm. I'll be off early Saturday and will have the evening to spend with my cars. I'll preform that test and report back with what I've found.