maybe a chemical reaction, i think the heatpipes are filled with some kind of "stuff" that circulates when getting hot and will bring heat to the outer areas of the pipes. like a refrigerator. Maybe a pipe is damagad and somehow producing a noise....okay, okay, very unlikely its that but where should the sound stem from if it has no mechanical components moving?
I once had the same problem, ripped of the hsf cuz temperatures was too high and i wanted to clean the cooler, when i suddenly recognized that i didnt have any thermal grease left. As a child i always watched this McGuyver on TV and i thought, maybe toothpaste wil do it! Dont call me a lier its true, though i would not suggest u to use it, except in cases of emergency which it was for me when it happened, i couildnt even order new thermal paste without the PC. To shorten it....it worked. didnt get good temperatures but it worked. With thermal grease i had 45° idle and 55 load, with toothpaste i had 55° idle and maximum 75° load. Dangerous it was. when the thermal paste arrived 2 days later i would go on with the toothpaste to test how long it works but after 3 weeks i got scared and better did it the way it has to be.
and concerning schadenfreude: ya, u got me sometimes iam relly envious when seeing the state of the art systems and looking at my crappy p4 northwood :/. but didnt mean to make u feel worse. when i build my first system - i was 20 already but had no idea what i was doing and all the advice of my friend who i thought was good in comps and stuff turned out to be fatally wrong advices...well, that was some bad days when i just couldnt get it to work, so i decided to give the comp to the repair service of a computer dealer and the guy would look at Comp and then at me and say: dude, u made everything wrong here, be lucky that it still works. Had to give him 20 bucks for his work and learned my lesson. Now the comp is 4 years old and iam ways better in building systems. useless story , i know but my point is, whenever i see people in the same situation i remember how i felt once and a smile comes in my face and in most cases iam sure it will end all good.
sry4offtopic