Hum and vibration when running empty or bobtail

My 2000 star has a nice little laundry list of things to be done to it and most of it are mysteries solved but in this case, I'm having trouble tracking down what I need to repair.

When I'm bobtailing or running empty, I get a vibration and hum that is rhythmic from the underside of the truck at highway speeds. When accelerating, there is no hum or vibration and when the cruise is applying power to keep steady speed up a hill, everything sounds and feels as it should. When running over flat roads however or when heading downhill on a slight grade, a hum that gets louder and then softer, louder and then softer, louder and then softer, etc...comes up.

I need to replace my torsion bar that connects to the rear diff' and I'm wondering if this might have something to do with the hum and vibration when not under load or if this might be a problem with the drive line and u-joints. If it's the second, how do I best check? Is it a simple issue of crawling under the truck when I'm greasing and seeing how much shake and give there is in the u-joints or, as is possible, am I just on the wrong track all together?

Cheers

 

So an update for anyone who may be interested in the future. The yolk connecting to the front diff' is loose and has far too much play up and down. I'm going to replace the Yolk and the U-Joint as well since I'll have it apart anyway. With this and the torsion bar to be replaced, I'll let every one know whether or not the vibration went away.

Cheers

 

I have to admit that I've spent more time on the couch in the past couple of days than on the couch but the little snooping about that i have done has almost convinced me for sure that in fact yes, it's likely the bearings. Unfortunately, I have literally NO MONEY right now. hahahaha. It's terrible. I was expecting payment from one fellah about a month ago but he has yet to come through because he's having financial problems all his own and if I scream and yell, it's not gonna do me any good and it'll probably just stress the poor guy out.

SO! Once I get some cash and finally get to figurin' this out, I'll let ya'll know how it turns out. Thanks again for the posts in this thread. they're all very much appreciated...even the one about the truck not knowing the words. No, you may not wanna quit your day job but thanks for the chuckle there bud. hahaha.

Cheers