Yer knockin sound behind the dash is most probably yer clutch or accelerator cable knockin in the partition where the window wiper mech and ECU sit. I replaced my clutch cable a couple of years back and was plagued by a knockin noise for a few days until I figured out what it was...
Yes, they are solid compared to most cars of the day, but you have to bear in mind that the earliest Mk3's are now 12 years old, so some of them are bound to have gained a few rattles here and there!