Chasing mystery noise.

[ss2115]

I need help with a terrible noise from the engine.
1995 850 basic model - B5252S 2.5litre Auto.
Only 157,000klms though.

Noise is best described as a mixture of a Kazoo and a sort of wheeze with a tiny bit of wet fart to it which makes it sound terrible. Not hugely loud - about a 2.5 on a 1 - 10 scale.

Only on a cold engine, and not in time with the engine but can be constant at idle or a fast idle and will come and go with throttle blips. Disappears above 1500rpm or thereabouts. Disappears after the engine has minimum warmth so isn't there long enough to reliably trace, but it sounds like its coming from under the valve cover.

I thought it was belt related. I noticed the tensioner had a rattle to it and thought it might be reverberating.
Replaced the tensioner, idler pulley and the serpentine belt. New belt routing.
Did not improve anything. But in really hot weather here (44 degree days) - the noise doesn't occur. Its only after a cool night.

The timing belt was replaced at 121,000klms.

So today I removed the main belt and ran the engine and the noise is still there. It distinctly sounds like its under the valve cover.

I looked up this forum and YouTubes and saw the information on the PCV valve. It was described as a sound similar with air being pressurised out past seals etc.
The engine has no oil leaks and doesn't consume oil.
Its idle is perfect and it doesn't lack performance.
The oil cap is easy to remove as is the dip stick. Oil level is spot on the line.

However, I did the disposable glove test over the oil filler (cap removed of course) and even plugged the dipstick with my thumb in case of a poor o-ring seal but it was all perfect - there's just a very slight vacuum that gently pulled the glove in.

So I don't know where to go with this next.
I wondered if the last service put a cheap oil filter on it and the top of the engine was oil starved. I've yet to look at the filter brand, but the bottom of the oil cap has clean oil on it, the camshaft I can see turning looks well coated, and little revs splash oil out the oil hole to the inlet manifold.
*Of course, it could be the exhaust side which I am unable to see.

Stethoscope tells me the front alternator bearing has some very slight noise, but AC and PwSt both quiet.
Haven't been able to use the stethoscope elsewhere around the engine with any good change of nature noise as noise has disappeared after three short starts so have to wait for another overnight.

But before I go to the extent of pulling off the valve cover and looking at the distributor drive and anything else I can inspect for wear, I thought to ask here if this is a recognised problem, or another unlucky owner has been afflicted with the same noise and knows the fix?