Skoda Superb 1.9TDi PD130, 131k miles

Ok the otherday i was driving with the windows down and noticed this odd noise. I will try describe it.

it sounds like some one clapping 2 10 pence pieces together between finger and thumb..... its a short duration, high frequency noise with that kind of metallic quality. try clapping 2 silver uk coins together, its a kind of plinking noise but its not sonorous.

i guess another way to describe it might be a sudden burst of high pressure air escaping through a nozzel or something.... for only a few milliseconds before its stopped. like if you had a leak in a pressurised air hose and you use your finger to quickly cover and uncover the hole....you get that repetitive psst, psst, psst noise.

im not sure what it is. it builds with engine revs under decent engine load...... im not sure if its a weeping valve, one of the injectors is beginning to go, something to do with the air handling system (turbo/intakes). the car doesnt seem to be any worse off for it though, its just an odd noise that i noticed...its not a normal noise and noises, for me, are often a way to diagnose problems.

any ideas?

Intermittent vacuum leak perhaps? I believe pretty much everything is driven by vac, EGR valve, variable turbo geometry, etc...