noticed this a while ago with my XJ, whenever I get my engine up to about 3k rpm and let of the gas a little the engine makes a very loud noise that sounds pretty much like a roaring. I don't know if it's normal or not, but if I just keep accelerating it won't make the noise, so I'm assuming it's not normal. I'll see if I can get a vid but it usually only does it if I am going up a slight incline around 35-40 mph at 3k+ rpm.

EDIT: The best way I can actually describe the noise is that it sounds like a Jake brake on a semi but at a little bit higher pitch and faster rhythm.

 

i had the same problem a few months ago with the roaring at 35-40 and it turned out my u-joints were bad so i got them replaced and havent heard anything since.

 

The U-joints? Never would have expected that. I'll have to check it out...but just in case does anyone else have any suggestions?

also, the thing is it only does it at a certain RPM when going uphill, and sometimes at a certain speed. If I'm on flat road and keep accelerating it will never make the noise

 

U-Joints all the way. Give everything a good shakedown, you should find the problem one(s) fairly quickly.

 

Yeah when my u-joints started going bad I guess the angle of going up combined with the loosening u-joints let then have alot more play so it caused the roaring. But don't make the same mistake I did. I let them sit for a while and they got loose enough where on the flat roads between 35-40 they would roar.

 

There is only 1 thing that wouldn't make sense though, I have just found out today that it only does it when the engine is still cold (I start the jeep, let it sit for about 30 minutes or so and then drive down the road. About a 1/4 of a mile is where I always hear the noise in the morning before school).

EDIT: accidentally put Seconds instead of minutes

 

Wtf are you doing 3k on a cold motor for?!

 

This. That is asking for spun bearings!

 

I shouldn't actually say "cold". It's only when I first start running. The temp is up around 210 which is the running temp (as you obviously know :P). It's only when I first start to drive and its only when I go uphill in a lower gear (automatic). It actually stopped doing it, so as of now I'm not worried until it starts up again. And if something does go wrong, I have a powertrain warranty (not sure when it expires)