[Kev314]

Mystery loud bang when stationary.

2014 328 xdrive at 51,000 miles.

Just back from store 10 miles away and unloading trunk.

About 30degF and drizzle:

Handbrake off.

Standing behind there was an extremewy loud and sharp pop/twang as if a spring gave way. So loud that I jumped back. Sound seemed to come from straight ahead, from within or around trunk. A person standing about 40 feet away thought something large had dropped.

Nothing obvious wrong. The trunk still closed and opened smoothly. Went for a test drive - no problems revealed. No shattered glass or deflated tires. Peering underneath, rear coil/springs seemed ok though hard to tell. No branches fallen on roof of garage.

Any ideas or things to check?

[WallySlash]

Huh? Seems like you checked all the big springs.

Some smaller springs in the rear of the car that may have made noise but not a big noise.

Maybe an emergency brake spring or brake shoe spring broke?

The fixing bow on the exhaust flap sprung?

Or perhaps your rear shock was stuck down and popped up? If it was icy it may have frozen and then gave way after you reduced the load in the trunk.

[Kev314]

Huh was my thought too! I like the rear shock idea, but it wasn't very cold and had just been driven for about 40 minutes, and load in trunk only ~ 20 lb of groceries. Conceivably from an external noise like sonic boom but directionally confused. Thanks for responding.

[COKen]

You may want to also visually check your suspension springs. If one broke, it may not drop the car on that corner, so it may not be obvious.

[Kev314]

When I get down on the ground and peer in the coils look ok. But, "It may not be obvious" - good point. Can't see top of coils and possibly broken up there. Maybe I can worm my cheap inspection camera up there.

[WillLowe]

Was it not one of the boot springs? There's 1 on either size maybe one popped out?

[agentorange]

The parking pawl wasn't fully engaged and the car moved to the next spot on the ratchet? Sorry, but I always cringe when I read "handbrake off" in these stories, just the way I was taught to drive.

[Kev314]

WillLow:

When viewed through the slots the springs seemed to move smoothly when articulating the lid. Conceivably one had been "hung up" somehow and suddenly released.

agentorang:

I hadn't thought of that. However, the sound was very, very loud. I've little experience with guns, but am tempted to say that it was like a nearby gunshot. Somehow I would think the incrementing pawl would not be that loud. Interesting idea though!