Engine rattle - 3500RPM or so

[JJorgensen52]

Guys, I'm pretty stuck on this one, so hoping somebody might have some insight:

1972 C3500 GMC, 350 with 4 speed, PS. No A/C or A.I.R. stuff. Does have the standard stove pipe riser on the driver's manifold

I get an irregular rattle coming from the motor above ~3500RPM ... best guess, no tach. It occurs under any load condition or throttle position, appears to be entirely RPM dependent - no noise at low RPMs, noise at high RPMs. Doesn't vary with RPM once it starts, at least that I can detect.

It sounds not entirely regular, like it's not a rotating part. I'm thinking potentially the heat riser pipe / manifold shield, or something to do with the exhaust? I can't seem to find anything, though.

The motor runs as smooth as glass, has 50k miles original on it, never been opened. Hardly burns any oil, everything looks clean inside so far as I can tell.

It will make it more or less continuously driving on the highway - I start hearing it at 65 and it doesn't really change with RPM above that.

Any ideas? Thanks in advance.

[JOJABOY]

Detonation maybe?

[67 chevelle]

Originally Posted by JOJABOY

Detonation maybe?

My guess also , put premium in it and check for vacuum leaks , have you messed with the distributor recently ?
Tuneup is suggested , oh and detonation or "ping" will destroy your engine in short order if not attended to

[TBONE1964]

This is under a load only? Will it happen if you just rev the engine up in neutral? You say it does it at 65 and over? What happens if you are going 68 and put it in neutral and let your foot of the clutch and coast?

The weirdest things cause rattles. A loose piece of trim under a load at the right speed will make a noise. I have seen brake lines buzz if they are not attached correctly. You may need to have a second person wit you to isolate where the noise is coming from. Sometimes what sounds like under the hood is actually back by the tail gate.

[JJorgensen52]

Originally Posted by 67 chevelle

My guess also , put premium in it and check for vacuum leaks , have you messed with the distributor recently ?
Tuneup is suggested , oh and detonation or "ping" will destroy your engine in short order if not attended to

When I got the truck 3 months ago I reset base timing to 6*BTDC (I usually use 8* but it idled better at 6*) and checked the vacuum advance, which is working as it should. I can try premium, plugs and such aren't brand new but they're not too old and they look good.

I've been avoiding revving it enough to rattle if at all possible, I've seen what knock can do

Originally Posted by TBONE1964

This is under a load only? Will it happen if you just rev the engine up in neutral? You say it does it at 65 and over? What happens if you are going 68 and put it in neutral and let your foot of the clutch and coast?

The weirdest things cause rattles. A loose piece of trim under a load at the right speed will make a noise. I have seen brake lines buzz if they are not attached correctly. You may need to have a second person wit you to isolate where the noise is coming from. Sometimes what sounds like under the hood is actually back by the tail gate.

It happens under any load conditions and in neutral - it goes away as soon as the RPMs come down, even at speed.

[67 chevelle]

Your distributor might be toast , or someone may have put the wrong vacume advance canister on , they make several , 6 or 8 degrees initial advance is defiantly not alot , sounds like stock . one cheap thing to check is get a avdance tape put on you balencer , and see what kind of advance you have at 3000 rpms , should be around 32 degrees and not climb after that , a distributor with alot of miles on it can sometime ware , and you can get too much advance

[Mr Chevorlet]

look at the bottom of your drivers side exhaust manifold and see if the spring tensioner is off of the heat riser or weak, i had this issue years ago and that was it. I removed the peice compleatly and went with straigt exhaust. Did not need it anyway because i changed carbs.