Odd Clunk - new sound

[psa55]

I've got a new, very odd clunking sound coming from my TB. The sounds happens on startup, before putting in gear, and typically after it has sat for the night. TB drives great, no issues after the clunk, after trying to monitor this for the past week. After the initial startup, it never comes back until it has sat for an extended period of time. The clunk appears to come from the rear passenger side, but I can't find anything. I thought about rear sway bar links, but that would cause a a repeated sound. It is A/C season, and see a post going back 14 years where this was related to an A/C dryer.

Can anyone point me in the right direction here with what I should be looking for, potential causes and/or repairs? Other then the loud clunk on startup with vehicle still in PARK, the TB performs as expected.

Your insight is appreciated!

 

[psa55]

I decided I use AI lots in my life, what about diagnosing this issue.. well here are a couple good places to look, all of which seem quite harmless.

The one‑time clunk on cold startup, from the rear passenger side, while still in PARK, on a TrailBlazer, is a classic pattern — and there are only a handful of components that can physically make that noise without affecting drivability.Short answer: The most common causes are fuel tank movement, rear suspension bushings, or the A/C system pressure equalization (yes, the old A/C accumulator/dryer “pop” is real).
Below is a breakdown so you can pinpoint it without guessing. Most Likely Causes (Ranked)

1. Fuel tank shifting — #1 cause of single cold-start clunksWhen the TB sits overnight, the plastic fuel tank contracts slightly.
On startup, the EVAP purge + fuel pump pressurization causes the tank to expand and “pop” against its straps or the skid plate.Symptoms match perfectly:Happens once, only after sittingComes from rear passenger side (tank sits offset)No repeat noise while drivingNo performance issuesCheckush up on the tank with your knee — if you hear a dull knock, that’s it.Inspect tank straps and rubber isolators for wear.Fix:Add rubber isolator material or replace worn strap pads.Tighten straps if loose.
2. A/C accumulator/dryer pressure pop — a known GM quirkWhen the A/C system equalizes overnight, the accumulator can make a sharp metallic pop on first startup.This is especially common in A/C season, exactly as you mentioned.Check:Start the TB with A/C OFF.If the clunk disappears, this is your culprit.Fix:Usually harmless.If loud, replace accumulator/dryer (~$60–$120 CAD).
3. Rear upper control arm bushings — often overlookedThe TB’s rear suspension uses a tri‑link setup.
When bushings dry out, they can “snap” into position on first movement — even while stationary — due to chassis flex from startup torque.Check:Look at the upper control arm on the passenger side.Pry bar test: if the bushing shifts suddenly, that’s your clunk.Fix:Replace bushings or the whole arm (~$80–$150 CAD).
4. Rear shocks topping out — less common but possibleCold fluid + worn shock bushings can cause a single “thunk” when the chassis lifts slightly on startup.Check:Look for oil seepage on shock bodies.Bounce test: listen for a dull knock.
5. Exhaust hanger shift — simple but sneakyThe exhaust expands quickly on startup.
If a rubber hanger is worn, the pipe can “jump” and hit the frame.Check:Grab the tailpipe and shake side-to-side.If it clunks, you found it.

What I’d check first (in order)Fuel tank movementA/C accumulator popRear upper control arm bushingExhaust hangerRear shock bushingsYour symptoms strongly point to fuel tank or A/C accumulator — both are harmless but annoying. Want to narrow it down fast?I can walk you through a step-by-step diagnostic, starting with the easiest checks.Choose what you want to do next:Check fuel tank movementTest the A/C accumulator theoryInspect rear suspension bushingsCheck exhaust hangersIf you tell me the year of your TB, I can give you exact component locations and photos.

 

[Mooseman]

Rear noise wouldn't come from the A/C since the whole system is in the front (you don't have an EXT which did have a rear system).

One thing it didn't mention was the driveshaft/pinion/u-joints. It's possible when the engine is started and it shakes that the driveshaft could move a little. A clunk could come from a dry slip yoke. I had that but while driving coming to a stop. Pull the driveshaft out and add some grease to the slip yoke splines and check your u-joints. Could also be slop in the pinion to ring gear.

 

[movietvet]

Just start and not moving? My guess is something to do with exhaust mounting.