need a diagnosis, clunking sound Options

[ghostdog]

recently my car has started making a strange clunking sound. this started after I put it up on jackstands to replace some of the screws in the plastic splashguard. now I don't understand how this could start a problem such as this but I thought I would mention it anyway.

okay so the problem is as follows:

starting up the car when I put the clutch in and turn the key it makes a loud clunk. when I accelerate suddenly, either when changing gears or just stab at the throttle this same clunk can be heard. the sound does not happen when the car is not in gear and happens in any rpm. my friend commented that it sounds like something is hitting the firewall, yes it almost sounds like that and can be quite loud if I jerk the throttle back and forth a couple times.

whoever tells me whats going on gets a cookie

thanks.