Probably is the exhaust pipe hitting against the floorboard.
1. Aftermarket exhaust system installed without enough clearance when made.
2. Could also be a broken engine mount on drvr side. Raise hood, apply hard brake pedal, put into forward gear, give alittle gas, if engine rolls up from mount, mount is broken.
3. Could be an exhaust leak, very small one.
4. Could also be the piezoelectric senser used for "hearing" the engine knock(pinging). Attach timing light, watch timing marks with t/light, tap block with hammer, t/marks should retard in timing, then return when hammering stops.