e60 M5 - constant speed misfire?

[Tony B]

Raising this question on behalf of a friend, who has a 2007 E60 M5.

He has owned it from new, covering 27k miles so far. No expense spared on servicing, and it is still under (extended) warranty.

His words:

"I have the following issue which is not being logged by the engine management system as a fault.
With engine running at fixed rpm anywhere between 3400-3600 rpm there is a slight "splutter" in the exhaust note every 1 to 2 seconds. If I hold constant rpm outside this range the exhaust note is constant."

My words:
I have heard/felt this for myself, and stood behind the car whilst he holds the revs mentioned. I can hear a quite distinct "woofley" sound from the exhausts, randomly from left or right exhaust pair.

The car seems to be down on power at the top end, as it was slow to exceed an indicated 160mph (autobahn of course) whereas normally it would run up to an indicated 180mph quite quickly.

It will be in the dealer's tomorrow, but they seem to be pretty incompetent in diagnosis, so any ideas gratefully received!