If you loose an air injection plug you should hear it clearly at idle (been there).
Since it is there at idle, you should be able to get close to it by jacking the rear of the car up and getting underneath while idling. Even run your hand along close to where the noise seems to be. You should at least be able to identify it to one side or other (unless it is central- fan).
From memory I have had broken studs that did not fall out when valve covers removed. I would suggest a magnetic pick up stuck down each stud hole to see if you pull anything out with it (the nuts are magnetic - but not normal socket type nuts. They are allen key barrel nuts).
Alan

Heard that sound as well......head studs......:(

I had a chance to jack it up today and it wasn't the sound of financial doom, but thankfully an air injection plug that had blown out, as TJJ911 suggested.

So I raced to the hardware to see if there is a 10mm bolt with a 1mm pitch, but no one had it, so I'll check out the nuts and bolts shop tomorrow.

From looking at it though I'm not to confident of the threads so I might need to put a time cert / helicoil in there.

Cheers for the help guys.

I think ther'se a mercedes part to be used for this (no sure wich one), I applied loctite on al the blocking off plugs, only one failed over time. Cleaned it out again/degreased and applied loctite and... no problems ever since. I can remeber also some of my port threads wera a little doubtfull.

Good luck with the repair!

hex head 1/8" pipe plug. the taper fit locks them in nicely.

Or there is a 914 brass plug that has something to do with the brake system from memory.
That is what I used.
Alan

914 rear inside caliper plugs. takes time to order and receive. the pipe plugs are down at your local auto parts store, fastener store, hydraulic store, boat store, etc.

TJJSC gets a gold star.

Good news.