Like the post says, my 09 Sierra hybrid will make a single loud metal sounding clank when the engine switches from electric to gas. It doesn't happen if I punch the pedal enough from a dead stop to start instantly on gas. Only happens when I'm accelerating from a stop and rolling on electric and it switches to gas. Sometimes it's not very loud but sometimes it's very loud. Has me worried thinking I should dump it at a dealership before something really bad happens. Anyone had this happen or heard of this?

 

how old is the aux pump?

check for broken motor/transmission mounts, loose/missing bolts, bushings,shocks etc .. it's pretty common on the GMT900, drive shaft barring might also cause this noise .....

 

Not sure about the aux pump. I’ve only had the truck a year. Forgot to mention it sounds like it’s coming from the back of the truck.

 

check the barring on the rear diff (ujoint), rear suspension, shocks,, springs and or loose Auxiliary muffler... also look in the rear and see if something is rolling around in the back on the SUV
maybe even a rock inside the tailgate (I have a coin the rolls around when making turns LOL)

small chance it's the U joint at the differential which is a "common failure"

 

Just rebuilt the differential on my 09 Tahoe. Not fun. The carrier teeth were broken on several gears.

 

I get the same noise and have had it probably the last 40k miles. I have chalked it up to the first cylinder not firing at startup or there being a small backfire of unused fuel in the exhaust when it first starts up. It is most noticeable when it switches out of full electric mode to ICE while cruising at 28mph. I have had misfires on other engines that darn near sound like rod knock and this seems to be very similar to that albeit only one hit of noise.