knockn noise??
 

My wrx has recently started making a noise (kind of knockn noise) when i drive off in 1st gear..gets louder faster i go,it doesnt do it in any other gears.I had a look underneathe and where the propshaft goes into diff it was was all wet.Would it possibly be low in gearbox oil? Jackd it up and went under it sounds to be coming from gearbox...slight knockn noise,when engage clutch it disapears? Any help much apreciated

hmmmm please help..........

bump^^^