Exhaust noise in carb

Hi Guys, does anyone have any ideas what the cause of what sounds like exhaust noise from no1 cylinder. The enginer is a beetle 1600 with twin carbs. The noise sounds a bit like a 'chuff' when the engine is about 180 deg from the firing point of no 1 cylinder.
The engine appears to run ok other than this loud strange noise. We took the engine out because of this and fitted new valves and lapped them and the barrels back in.
We have been told it could be a weak valve spring?
Has anyone heard of this or being able to replace the spring without having to remove the engine again?

Your thoughts would be appreciated