CLK320 Squeal

[PhilosCLK320]

Hi Guys!
Newbie Question here....just bought a UK Spec 1998 CLK 320 Sport with a genuine 48K on the clock at my local car auction. Car has run fine all week until earlier today when as the temperature dropped and the air got damp an annoying squeal can be heard from under the bonnet (hood). Now, you can only hear it with the door open or the window down and it fades until almost gone if you accelerate hard - after 20 mins or so it seems to go altogether. If you start the car and leave it idling for 10mins it goes away altogether, however if you switch the engine off and leave the car an hour or so the sound comes back. Took it to a local dealer who tightened a belt (don't know which one ) and it was gone for an hour or two...noticed it came back tonight when starting from cold. Took it to a mechanic friend of mine who said the belts were fine and he lubricated a few parts and the squeal was gone. Drove home...no noise...parked up, left it 2 hours...popped out and re-started and you guessed it...noise is back. Want to do about 300 miles over Xmas period but don't want to damage my new pride and joy. Any help greatly appreciated!