What noise? The tappet noise of the chuffing noise..
Former is a tappet
Latter, yes, sounds like induction or exhaust noise from running on 3 cylinders...
If you'd revved it a bit and held it at say 2,000 perhaps we could tell...
PS. the first thing I'd do on that engine, is check/clean the plugs leads and all the intake connections/vac & breather pipes
If it doesn't idle any better
then take the compressions
I bet one of them is down then...
===OTOH==
On the other hand it could be that tappet making it idle so bad, but whatever, it is idling too slow, and it doesn't do an engine any good to struggle along like that for very long.
(I'd still take the compressions, just to be sure its worth spending time elsewhere though, do it once and at least you know)