2004 Hummer H2. I can’t seem to find an answer for this one. There’s a rattling sound coming from underneath the truck around 1100 rpm or 1600 rpm, but only when it’s in gear. It sounds more like a heat shield rattling than anything, but nothing seems to be loose under there, plus I would expect a heat shield to rattle even out of gear. It does seem to be coming from the passenger side, just behind the front seat, which is where the exhaust runs, but still doesn’t quite add up for me. Any ideas?

 

You have an engine warning, do you know what it is?

 

Yes, it’s because the catalytic converters have been removed.

 

That rules out a loose cat.

The heatshieds on mine were rotted through, ended-up pulling them both off.

Also had a rattle under certain conditions, eventually found a 1/2" socket wedged in the chassis, one that I'd lost ages ago.

 

I crawled under it looking at every heat shield I could. I found one that was a little bit loose, but I tightened it up before I took that video. I’m concerned it could be U-joints. But it’s strange to me that it doesn’t really matter how fast I’m going, it just does it around a specific RPM.

 

A rattle that occurs a certain frequency is usually because the conditions are just right to start oscillating, like pushing a swing or the pendulum on a clock say. The load and position of the engine may also play a part in it.

It suggests something is loose, maybe a bolt in the skid frame, skid guard or transfer case guard, even an exhaust bolt.

I had a "canary" that sang only a certain revs, it turned out to be a serpentine belt tensioner bearing, turned off the A/C and it went away. Things like that can be hard to track down.

Another pair of ear's might help with the window down.

Is the rattle more noticeable just after you say "drive" around the 48s mark?

If so it does sound metallic, like a bracket that's loose, or rubbing/tapping against something.

Hard to say of course.. a rolling road would be good.

 

Yeah, in the video, I shift it into neutral for a moment to show that it doesn’t make the sound out of gear, only in drive. I’ll try to hunt it down.

 

Have you checked your front differential crossmember bushings and bolts? What type of shape they are in?

 

It really sounds like it’s coming from behind the front seats, but I’ll take a look at those. I’m planning to crawl under there with a socket set and tighten every bolt I can reach, whether they seem loose or not.

 

maybe one of the skid plates are loose or missing a bolt