Mystery thud, 06 Express van

I have no real expectations of finding an answer.  This seems like something that a dedicated GM Van forum should tackle, but there aren't exactly any of those with a good S/N ratio.

About 60% of the time when I accelerate from a stop, I get a thud.  Not really audible, more tactile.  I describe it as if you had a basketball in your trunk and it was gently hitting the back every time you took off.  Very faint.  It always happens during normal acceleration.  It sometimes happens about 1/2 second after you start accelerating, sometimes it happens immediately after you take your foot off the brake.  Sometimes it doesn't happen at all.  I thought maybe it was fuel sloshing in the tank, but it happens the same regardless of whether or not it's clear full or on fumes.

So here's what I've checked so far

• rear axle/suspension bolts/bushings (thinking axle motion)
• spare tire cable tightened to make sure it wasn't moving
• removed jack from storage and set it on the floor
• removed all the dead hookers
• made sure caliper bolts were tight and calipers weren't moving.

I thought it could be some kind of transmission issue (I am getting an occasional slow 1-2 shift), but it feels like it's coming from the back... although I know that doesn't mean it is.

Put on your psychic caps and help me figure this one out.

Does it have a carrier bearing?

I say carrier bearing or trans mount. This one is 4x4, right? Does it have a double cardan or cv in the driveshaft?

Muffler hangers all good? Them stock GM mufflers are YUGE.

In reply to Curtis73 (Forum Supporter) :

Driveshaft/U-joint slop?  

Trans mount/engine mount?

It is an AWD.  I'll look again, but I think it's just a one-piece driveshaft with regular U-joints.

Muffler hangers good as of the last time I looked which was August, and this problem started in the spring.  It was a NC van, so next to zero rust.

In reply to John Welsh :

Possible, but I would think there would be other symptoms.

Does it also happen uphill or downhill ?

Just thinking it could be something rolling in the side sliding door  or  rockers.....

I have a 2005  express.....

There is some magic spline grease to put in the slip yoke.   Sometimes it binds coming to a stop, and doesn't plunge properly until you take the brakes off and cycle the suspension a bit.

Easy enough to try, anyway.

You removed the dead hookers. What about the live midget one?

Dusterbd13-michael said:

You removed the dead hookers. What about the live midget one?

She's pretty securely taped to the seat.  Doubt it's that.

californiamilleghia said:

Does it also happen uphill or downhill ?

Just thinking it could be something rolling in the side sliding door  or  rockers.....

I have a 2005  express.....

up/down hill doesn't make a difference.  Does it at any angle.

Streetwiseguy said:

There is some magic spline grease to put in the slip yoke.   Sometimes it binds coming to a stop, and doesn't plunge properly until you take the brakes off and cycle the suspension a bit.

Easy enough to try, anyway.

This.  I have used regular moly paste with good results.

I had a rear spring shackle that would bind a bit and release upon acceleration on my Tundra. Had to disassemble/grease/reassemble and it went away.

Any different when going forward after reversing? If it's not unsecured dead hooker parts, and if you're sure the midget is adequately restrained, we're really running out of options.

AngryCorvair (Forum Supporter) said:

Any different when going forward after reversing? If it's not unsecured dead hooker parts, and if you're sure the midget is adequately restrained, we're really running out of options.

I mean....hookers and midgets are the first thing I look for when trying to figure out what a weird sound is.

After that I'm assuming wizards or some sort of demonic presence. 

Then after that I thought the carrier bearing. I just skipped to carrier bearing over the obvious answers that Curtis had obviously already checked

Receiver hitch ball mount or cover??  I have an '06 and sometime if I leave the ball in the receiver I can hear it bang a round.

Is the rear door latch adjusted properly? 

Has something gotten inside the quarter panel?  I worked at a GM dealer years ago and we had a new Delta 88, told ya it was years ago, anyway the owner complained about a "thud" in the back. After several days and test drives, they had the oil change guy ride in the trunk. Turns out there was a coke bottle wrapped in a shop rag behind the quarter panel cover in the passenger side of the trunk, with a note saying, "Ha Ha You finally found me" .

Also go back over your frame to body bushings and make certain that one hasn't left the building causing a small amount of flex related pop.

I’m getting rear differential vibes.

What does the fluid look like?

My first thought is driveshaft moving around. 
 

well, after unsecured midget hooker....

Often it'll be the quadriplegic hookers rolling around instead of the dead ones that just kinda settle in one spot. 

Put me down for motor/trans mount, likely lower rear, if that's not the case. 

the two common Chevy issues causing this clunk are dry driveshaft slip yoke and some years had incredible self loosening axle/spring i bolts. 
 

check both.  

I have it too in my CTS, turns out it is the electric limited slip kicking in

I've seen a rear gear set "wear out" and the backlash increased to a point the pinion and ring gears would have enough space to "slap" together causing a noise.

Since we're mostly guessing here I'm going to speculate that it's something wonky in the transfer case.

I really want the answer to be "unsecured midget hooker stowaway"