Funny noise from engine.

[W111AAM]

I'm going to attempt to explain these engine noises as best I can in the hope that you can help and advise me.

I initially noticed the noise on the way back from Le Mans. When accelerating, under the noise of the exhaust you can hear the engine as it builds speed making a bum bum bum noise. I thought it sounded as if one of the cylinders wasn't firing. This continued for an hour or so, until I stopped to refuel, after which the noise didn't exist anymore. I did wonder if I’d had some dirt in the fuel, or some dodgy fuel?

Then, last night I noticed it again. I can only make the sound out in a built up area where the sound bounces back of the buildings. It’s definitely not a constant sound as it’s only occasionally happening.

Also, when I turn the engine off (and I don't blip the throttle when doing so) there is the sound of something spinning for a second or 2 after everything else has gone quiet. This again in the last week or so has got far more noticeable.

Are the two issues related? In advance I appreciate you help on this matter.

[OscarT]

If it keeps you amused then why fix it? A good laugh is hard to come by these days.

Seriously could it be your rad fans?