Strange noise at start-up
 

When I turn on the engine first thing in the morning I get a noise I can only describe as
that of a lawn mower engine. It sort of keeps sounding like that for 10 minutes after
starting to drive the car, then disappears and won't happen again during the day.
Have reported this to the dealer and even gave him a short video of it showing the
open bonet, but they tell me the engine doesn't give any errors and that they don't know what it could be. Any feedback much appreciated. Thanks


Jo

It could be the auxillary air pump. I assume the M5 with a petrol engine will have one and it will run for a few minutes while the engine warms up.

Thanks for your reply NickC. Been doing some searching and found this:

http://www.youtube.com/watch?v=39Bh8ooji0Y

It's almost identical to what I get and the poster of the video also mentions that
this happens at first start but not always, that is, it's intermitent.

If after watching you have any further comments, I'd be very glad to hear them

That sounds like the auxiliary belt. When you first start up the battery loses a load of charge and the alternator tries to replace it. This puts extra load on the belt and it slips slightly causing the "chattering" noise. Once the belt warms up and the battery is charged the noise goes away until the next cold start. Try spraying a some silicone lube onto the belt while it's making the noise and if the noise stops you know that's the cause.
It can be quite difficult to permanently cure but start with replacing the belt and making sure all the pulleys are clean.

NickC

I am extremely grateful for your answer will do as you suggest. I take it
this is not a serious matter and that I can live with it without fear of
engine damage.

As long as the diagnosis is correct then it won't cause any problems in the short term, but I'd have the belt changed at the next service.