when i had a classic...i had a bit of a panick, mine developed a knocking noise, sounded like it was coming from the driver side of the engine, turned out to be the middle silencer internals had come loose....worth a check....de-cat centre soon sorted that out[]
worst case scenario, is the bottom end has gone, or little ends gone on 1 or more cylinder.
edited to add: possible piston slap....also. overfilling wont cause high oil pressure..definately wont do it any good though, classsics prefer to be just under the full mark.
a bit hard to diagnose without seeeing and hearing it.
get it checked out pronto.
one thing...what mods has it had done that you know of?.