Well, it could be a lifter, or it could be "rod knock", a very bad
thing.  To tell which it is, disconnect each spark plug wire in turn
(engine off, then start it again) and see if you can get the knock to
disappear.  If it does, it's rod knock due to a bad rod bearing in that
cylinder. Stop driving immediately and rebuild or replace the engine!
If it does not stop, it's a lifter.  See the archives for many solutions
to this problem.

Good luck!
--
Stuart MacMillan
Seattle

'84 Vanagon Westy (Escape vehicle)
'65 MGB (Daily driver since 1969)
'74 MGB GT (Undergoing restoration)

Assisting in restoration:
'72 MGB GT (Daughter)
'64 MGB (Son)

Parts cars:
'68 & 73 MGB, '67 MGB GT