Extremely odd knock/sound

Ok pretty much EVERY day for the last week, ONLY at start up, and ONLY 1 time each day...

I start the car it runs for a sec and then BOOM (sounds like some one hit a bass drum) then nothing. Every time I hear it I fear my tranny just fell out.

I can feel it through my elbow sitting on the center console, I can feel it on the steering wheel, I can feel it through my seat, I can feel it slightly through the brake pedal.

The first few times I thought it was the E-Brake was over loaded, but I tried the last few days with the car starting foot on brake e-brake off. This always happens out of gear at start up.

Starting tomorrow I will start the car in gear with clutch in to see if it still happens.

Any ideas on what this could be?

I can't tell if the sound is coming from the rear or the front. I thought maybe the drive shaft U-Joints were about to go, but that wouldn't make sense, the car is in neutral so it's not spinning...

I'm very scared though.

lmao, no I know what a backfire sounds like, it's not an explosion, it's a quick THUMP, 1 time, 1 time a day, only at cold start up.

No accessories are on, trans is in neutral, foot not on the clutch, e-brake engaged, not gassing it, only thing I'm touching is holding the brake down.