Since yesterday I noticed this flapping/knocking noise under modest acceleration at around 3k - 4k rpm. It seems to be from the driver's side engine bay area and the frequency doesnt seem to vary with rpm. I can also feel the pulsing from the clutch pedal when I'm letting out the clutch during up shifts.

I noticed this noise right after removing the rear transmission mount spacer and the guy who did it told me every bolt is back on tight and I can't seem to relate this noise to the rear transmission mount.

any idea what it could be?