I picked up D2 that's super clean with only 85k miles on it. It's got some issues, but nothing major. I have come across a bit of a mystery problem though.

After 70mph, I have booming sound. Like a constant drone that sounds like a giant subwoofer constantly booming. The sound goes away off throttle, but loading the engine causes it to boom, louder with more load.

The sound is not exhaust, it's all factory. I thought maybe it was driveline. The truck has a new looking front driveshaft which I believe is the Lucky 8 shaft. And, there's not really a vibration.

My next thought is motor mounts, that seems to fit the problem more than anything else, but it passes the old brake and rev the engine test, no thunk from the motor.

Anyone else ever experience this? My 2003 did not have this problem.

 

My truck has a bit of a drone. I think because the PO busted the innards out of the cats. Definitely makes a difference in the exhaust note. I checked against a Y-pipe with the cats intact.

Could it be an exhaust leak? Possibly a drive shaft issue? Can you measure u-joint temperatures after a drive?

 

There are no exhaust leaks, but the more I drive it, the more I'm convinced it's exhaust related. I have no idea if the cats are hollow but I doubt it. My 2003 didn't drone like this but maybe this is 4.0 vs 4.6 thing.

I went over the truck today and checked the motor mounts, t-case mounts and they look new. It's amazing how good the condition is on all of the bushings under this thing.

 

Bent drive shaft unlikely if it has not been off road, but could be unbalanced, or a u-joint going. Make sure the drive shafts are lubed well. Poorly lubricated drive shafts are fairly common since they need more frequent maintenance than a regular car.

 

I did lube the driveshaft today. It's the Lucky 8 shaft and it wasn't too dry. Seems to be in good condition. I also had the tires rebalanced today which took away any vibration I had. I think this is an exhaust drone. I'm going to check and see if the cats are hollow next.

 

Two things. First, don't neglect the rear drive shaft and the slider portions for grease (you only mentioned one). Second, telltale signs on my cats was welded patches.

 

Good call on rear shaft. I'll grease that up too and check the rotoflex as well as cats tomorrow.