Does it happen by chance, just when you initially shift the clutch, like the first few 1000 revs?
Like a creek? Could it be from your exhaust area? I.e. below the window?
I had that problem, creek would drive me nuts, everytime i rolled a little on the clutch it would happen.
Turns out, after i got it on a hoist, there was a bolt somewhere under there at the back that was SO SO SO slightly loose, tightened it up, never had a problem.
If its from INSIDE your car, no idea dude.