Hi
It must be the season for it. My '01 S500 has started making a whirring sound, which stops as I slow to idle.
I originally thought it was the cooling fan motor, because when I spun the fan it felt choppy. I took the motor out, squirted it at both ends with WD40 and allowed it to soak a few times and when I spun the motor it was then running freely.
I still had the noise though, and I've just taken a torch to it and there is some play in the ribbed belt tensioner pulley, so maybe I'm getting some vibration from that.
Try spinning the fan by hand, if it's not smooth then oil the fan motor bearings.
Also try waggling the plastic pulley on the engine belt as a source for your sound.
Tools required 10mm socket with short extension or 10mm spanner, small flat bladed screwdriver, WD40 with the red tube, hot beverage of choice to fill out the waiting time.
You'll have to take the fan motor off the fan framework to properly let the oil soak in.
However since the fan is belt driven it's best to take the framework out, or at least undo it for better access.
1. unplug electrical connection - it has two tabs that need holding back to release it.
2. remove shroud over fan motor by prising up the top of the two retaining pins
3. remove air ducts from front of car to air filter housings, these just slide off
4. undo the two 10mm bolts that hold the cooling fan frame to the front of the car - it might be worth spraying these with WD40 and leaving for a few minutes, one of mine sheared off due to the reaction between the steel chassis and aluminium frame
5. lift the fan frame straight up, about 50mm - it won't go any further because of a coolant pipe and oil cooler pipe
6. lean the fan frame back towards the engine - you will be able to see the belt between the motor and fan pulleys
7. use a short screwdriver to pull the belt to one side of the large pulley and rotate the fan so that the belt comes off - catch it before it drops down onto the undertray
8. undo the three 10mm bolts holding the fan motor onto the fan frame - WD40 them first
9. stand the motor on its back end, spray a small amount of WD40 onto the shaft between the motor body and pulley (use the red WD40 tube so as not to soak the innards of the motor with excessive amounts of oil), let it run down and sink in using gravity, spray it again, and again, just little bits at a time, spin the pulley between sprays, depending how badly damaged it is you will feel the motor start to turn freely, and if there was any squeeking this should go away, repeat the spraying and spinning until the pulley and shaft spin freely
10. the bearings at the rear end are covered by a piece of paper, removing this gives access the the end of the shaft, spray and spin a few times as number 9
11. once you're happy that the motor is spinning freely refit it to the frame with the three bolts - it may be worth re-connecting the motor power and starting the engine to listen for any noise (make sure it is fastened to/suspended from something securely so it doesn't drop into the engine grooved belt, and obviously drop the fan frame back onto it's lower supports so it doesn't foul anything)
12. refit belt by putting onto small pulley 1st then aligning on large pulley and spinning the fan to re-fit it (like a bike chain that has come off)
13. refit fan frame, dropping it into bottom supports and re-fixing both 10mm bolts
14. refit fan shroud
15. refit electrical connection
16. refit air ducts
Did mine this morning in about 30 minutes, most of that time was letting the oil soak and spinning the motor, to make sure it was properly worked in.
Was loads quieter when I started it, but it got quieter over the next few minutes as the oil worked round better.
On my car it would appear that this problem was in addition to a failing grooved belt tensioner, which I didn't spot until after dark this evening.
That would work too, and is obviously the best recommendation, rather than just squirting it with WD40.
When I looked on ebay I couldn't find the motor separate to the frame, and the cheapest frame was £300.
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