BMW 316ti E46 N42 air leak?

I realise that the N42 is not everyone's favourite engine, and I am aware there are dedicated BMW forums but thought I'd ask in case anyone
here has come across an obvious fix...
Are there any common places for the 316 with the 1.8 N42 engine to develop air leaks? Doesn't appear to be an obvious leak from a hose, that does not mean it isn't, just that we can't find it... Intake manifold seals have been replaced.
Engine is "hunting" on idle and live data shows it is going in and out of closed loop. Thinking it's an air leak because there is a faint "whistle" (not a PWM whine) but can't locate where it's coming from. Anyone who's tried to work on one of these will know it's pointless using the normal methods to track down a leak - you just can't get at anything without significant disassembly And then of course you can't run it :(
Lift the revs slightly and it's fine.
One clue is that resetting (ie clearing the learned values) of the eccentric cam makes it run better for a while until it slowly gets worse again to the point where it finally stalls and is difficult to restart. Engine is on around 60K, VVT motor seems ok, no obvious play. No codes.
Background:
BMW 316ti E46 N42 history of rough running, recently replaced head gasket(+head skim and eccentric cam set up) because of excessive coolant loss. Also new chain+tensioner+guides and plugs along with coil packs. Previously lots of misfire and cam timing codes. Lower part of chain guide was broken, which is apparently a common failure on these.
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