Help:2000 Jetta makes strange vibrating noise

My 2000 Jetta standard transmission just started making a strange vibrating noise after I start the car. Last only a second or two. It does it if I have the clutch in and rev the engine - rpms come back down and it happens again. Also, there has been a knocking sound when I start the car too. Does anyone know what this might be and how to fix?
Thank you.