have you tried applying little lubricating oil to original fan? that's what I did with mine (just a drop) and its quiet now. its a temporar solution though, oil won't last long in there.
I think some people removed fan and even the heatsink and put one smaller just on PPC chip. (see album here)
but its not recommended because other chips need cooling besides the CPU. thats why it might be wise to keep the original fan (you can't get air through heatsink to cool all chips without punching holes).

Getting a fan of those dimensions at the right voltage is difficult. Most 40 x 40's are 12V thesedays. Those on the PPC are either 5V or 3.3V. I can't remember to be sure.
To Re-Oil your noisy fan, simply remove the Fan from the heat sink after removing the whole Fan/Heat-Sink assembly complete! ;-)
On the back (Exhaust) side of the fan there will/should be a small round sticker. By very carefully lifting this with a Scalpel or sharp Razor blade, you will expose the end of the Motor Shaft & retaining washer. This is where you need to apply a drop of Oil.
Best method is to use a needle or long needlework pin dipped in oil. All you need is one drop of very light Machine oil &  then spin it up to work the oil around the motor shaft & bearings whilst keeping it the same way up as you had it when applying the oil. Try not to get oil on the surface where the sticker goes for obvious reasons.
If your Fan already has no sticker, make one from Electrical tape after lubricating it. It shouldn't go anywhere once applied back to the Heat Sink.

Good Luck.  :-)

Yes, I looked at Amigakit. But the price is a little high: will cust something like 45 dollars with the delivery for my country. And I don't trust in this little coolers fans. Someone works very little time. I bought more than 3 40mm x 40mm for my USB case and in little time (some hours) they turns noisy. But here the cost is something like 1 dollar each. So is cheap to try. I only need to know if putting a cooler fan (PC fan with low profile) as stated in the beggining is adequate. Anyway thanks for all the attention.

Thanks

ddniUK wrote:
IIRC Amigakit has them.

I will look if this is possible with it without removing the big dissipator.

Thanks.

orange wrote:
I managed to put some oil without removing anything! IIRC, I used a small 'tube' from Cotton_swab or "Q-Tip" after removing the cotton. you only need a tiny amount of oil. but you should be very very careful because it might be conductive and in some other ways bad for PCB. (slowly spin the fan few times manually)