sometimes it happens when I start first driving in the day, something like hit the car from bottom - "BOOM" and it sounds like compressor start to work, but A/C is off, it just happened when I start to step gas.
it has 50K miles and 05 TL. do you think it might be the engine problem?

 

could be the ABS self test that you are hearing, others have reported a similar sound

 

is that normal? but it's really loud, like hit a stone under middle of ahead vehicle. if ABS itself, should I change all the rotors and pads?
Thank u pohljm for replying.

 

Many report hearing a fairly audible clunk that is attributable to the ABS self check. personally I rarely hear it on my vehicle. Hopefully others will chime in here. No need to change rotors or pads until they are at their wear limits I dont think that is the issue. Also on this car the air conditioner compressor will run for just about anything. defrost etc not just ac so it could be the compressor if you punch the defrost button