91 940GL with B230F

When the engine is cold there is a knocking sound from the engine bay. The sound alters speed with the engine speed ie. higher engine speed faster knocking. The odd thing is that it will do it for a few days then stop for a couple of days then start doint it again.
I've noticed some white deposits around the flange between the exhuast manifold and the downpipe (twin pipe?). My guess is that this is the result of condensation in the engine leaking out this joint. The engine does blow quite a bit of white smoke when it is cold, particularly when accelerating, but the coolant level is fine. I have felt around the joint for puffing when the engine is running but I can't feel anything. Could the gasket have blown? Should I be able to feel something if it had blown?
The sound gradually disappears as the engine warms up but is still sometimes audible for a little while after it has fully warmed up.
Any suggestions?

thanks
Brad