Today start to hear kind of ratling noise in the engine compartment. The noise won't die down until a few seconds after ignition shut off.
Later this afternoon, opened the hood and hear & feel the source is coming from the fan in passenger side.

Is this the bearing? What cause it? Please advise. Thanks.

 

The finding of noise is from right hand side fan's bearing. It has a bad spot thus the noise. The part no. is 8E0959455N 200W 300mm
Online price - Valeo brand - cheapest is $189.00 (free ground shipping) in DBC Tuning LLC - OEM & Aftermarket Parts for European Automobiles

Garage quoted me its cost price $288 so I save $99 by waiting a few days delivery from online ordering. Fan is still running, just noisy. Hope the new part arrive before the old one dies.

The left (driver) fan is larger size -> 380mm

 

I had a similar problem and it required a replacement of the aux fan motor - one of the fans had something holding it from spinning and caused a failure. So, when it got really hot, the other "working" fan would spin to keep the car cool even when the engine was turned off (it was sort of a stand by mode).

Anyway, I had to have the aux fan assembly replaced - Part Number 4ZN959455. Once replaced, the noise went away. Now, I try to let the car idle a little before I turn it off.

 

Part No. 8E0959455N 300mm 200Watt ... I got new one from UPS yesterday afternoon and will have the replacement done on Monday.

DBC is in back-order, so has to order from another source, same price + shipping.

 

radiator support. You then have enough room to work on it.

‪Audi A6 1998 - 2004 Front Lock Carrier Service Position (Radiator Support)‬‏ - YouTube

 

My aux fan replacement done in 1.5 hour today. After the above video, you have enough space to remove the fan housing (fastened by large and small screws), and then take the old fan out (fastened by 3 screws), install new fan in, spliced the 2 wires and reverse the above video procedure.