06 Grand Prix 2 thumps upon starting engine, near feet

[PontiacGranPain06]

When I started my car a few months ago it would make two thumps in quick succession which would cause vibrations near the right side of the driver foot well area from what I could tell.

Drove the car a few months the thumps never happened.

Got new tires yesterday and tried starting the car. Made thump noises, turned engine on/off 4 times and thumped twice each time in same area.

Drove car home about 5 minutes away. Tried starting engine and no more thumps happened once home.

No problems whatsoever while driving around though. Only happens on engine start.

Any idea what causes this thumping?

[Fivefingerdeathpunch]

Have someone start it while you watch the motor, maybe it's shaking around from a bad motor mount.

Or just check them with your eyes, the drivers side will be a nasty mess if it's all smashed up.

[PontiacGranPain06]

Would a motor mount only shake a couple times and then stop? Or does it also only sometimes thump a couple times?