I second checking the welds on the exhaust heat shields. One of mine broke and made a rattle under load that sounded just like engine knock.

 

man i hope i have that problem too, and NOT engine knock. my unlimited has 11,500 miles or so, and i've been hearing the "marbles in a can" when accelerating from a stop, or having to accelerate getting onto the highway (under a load as stated by OP). the other day getting onto the highway it sounded so awful that i wanted to just get off of the highway..just not a pleasant rattle sound..its funny because ive been wondering if anyone else had heard these noises under a load. guess its happened before. dealer thought it was just exhaust noise that im hearing.

 

I was driving up a short moderately steep incline on the weekend and suffered a brief 2-3 second interval of rough sounding dry upper engine, valve rattle. I even had to let off the gas to verify.

I got immediate flashbacks to my old 88 YJ I6 with 300,000 miles.

Has anyone else lost lubrication capacity in the 3.8l on an uphill?

Oil level was perfect, 600 miles on ODO

 

anybody know EXACTLY what the rattles could be? Mine only happens when im accelerating at a pretty good clip (really noticeable from a stop). all of the noise is coming from up front. it does sound like "marbles in a can." i hope its not engine knock.

 

Exactly is hard to say on any vehicle , especially over the internet. You just gotta try and narrow stuff down. Most common is exhaust heat shield rattles. Start there.

 

did you go to bigger wheels without a regear? will the same noise happen if you do a downshift and up your revs? If preignition you may get away with running a combustion chamber cleaner and a good hard run with higher revs to clean her out. (Italian tune up, redline a few gears)

 

thanks mkjeep..yeah i had a feeling about it being a heatshield somewhere..that's what it sounds like to the ear..and it only happens at certain RPM's..leading me to believe its not the engine, and thats it's being caused by the vibration/noises from the engine reving up. ill get under there and check it out tonight, mention it to my mechanic guy at the dealership tomorrow..hes one of the "good ones" and always replaces or fixes any parts i doubt w/o question...he's the MAN.

 

Get a rubber mallet and tap around on the exhaust, it'll help find it. Took me three days to figure out where it was coming from on mine. That crossover pipe was my prob. Have him check all your u bolts on the exhaust too

 

Ugh! I've been having this problem too. I am going to check out the exhaust tomorrow.

Is that a good thing to do? I've never had this problem, so I am not sure.

 

it's an annoying problem isn't it? it makes my motor sound like sh*t.