I fired up the truck to get it away from a tree during the storm last night and it had developed a new rapping, tapping sound. I thought it might be top end, but couldn't find the source. It sounded louder underneath and maybe as if it was coming from the cat. It wasn't an exhaust leak (I have one of those, too, and it's a different sound). This was more of a rattle or rapping sound that was in time with the engine speed at idle and went away when I revved it. It wasn't the usual heat shield rattle/buzz, either. It really sounded mechanical since it was a consistent tap-tap-tap, but who knows?

Any ideas?

 

As much as I don't want to say it, rod bearings.

If its coming from the bottom of the motor, and its a consistent rattling/tapping noise at certain rpm ranges, it could be the rod bearings.

I hope I'm wrong though.

 

Tap-tap-tap? Sounds like a lifter that's losing pressure. I know, because I have one that taps from time-to-time and I haven't taken the time to replace it. Been going on two years since I rebuilt the engine that it's been tapping.

 

I'm 90% positive it's not internal. It's not a knocking like a rod knock, and it doesn't sound like it's coming from the top end. The fact that it goes away when I rev the engine suggests something non-mechanical, too. A rod knock wouldn't go away at higher RPM, and a collapsed lifter would still tap, no?

It really does sound like the cat, and when I was under there, it was farther back--I was actually thinking transmission at first. Also, it just started suddenly. Last time I drove the truck, no noise, worked perfectly. Started it yesterday after a week of being parked, tapping started immediately. I have good oil pressure, synthetic oil since new, 75,000 miles--an engine like this shouldn't spin a bearing the way I use it. I don't think it's ever been over 4000 RPM.

I'll do some more investigating when I have a chance to get under there. Keep the suggestions coming!

 

U-Joints? Shift from D to R and if you hear a pronounced "CLUNK", they're worn.

 

U-joints are new and this just happens in Park at idle. Drat.

 

Then I defer you to my first answer of it being a lifter. The second item I'd suspect is a sticking injector. Do you ever run some Marvel Mystery or Berryman injector cleaner through your system? I do it every few months as lubrication of the fuel system is essential.

 

Hey matt, check the exhaust bleed tube on the passenger side manifold to the e.g.r. valve. they do rot out... small hole will sound similar to lifter tap at low rpm. If it isnt the tube you may have a warped or cracked exhaust manifold, look for carbon tracks where it bolts to the block or the throat area where it dumps into the y-pipe they will leak when cold but hush up when they get hot/expand. this is common for cast iron especially tow or service trucks that see more stress. hope it helps... carnutt

 

If it is the original catalytic converter
it very well might be covered by the official emissions recall
for the 1996+ Rams
so check with the dealership
for a possible free replacement

the heat shields attached to the firewall behind the engine
have also been known to make a knocking noise
at particular rpm ranges

 

Thanks, guys. I'm going to go ahead and have my shop put a new Y-pipe on it and see what happens. I do have a tapping exhaust leak on the driver's side in addition to the new sound on the passenger's side, so hopefully the Y-pipe will cure that. If not, I expect the manifolds are warped and I'll have them check for these other problems as well. It's just money, right?

If the manifolds are warped, should I replace them with a set of headers?