Might find that it's swapping between FM and DAB. I have this issue on my M140i and found that this is what was happening.

Edit: happens when DAB signal is lost.

 

Thanks, very annoying, might try using FM only.

 

Thanks, very annoying, might try using FM only.

If that is what is happening you can turn off the FM auto switching, it may then mean you have certain spots on your route where the DAB drops out for a few seconds , but on the whole it should give a good signal, maybe you have a DAB antenna fault if it's doing it excessively or where you'd expect a good signal, the FM switching is normally pretty seamless, but this will all depend on the strength of the channel in question, as some bigger channels have better coverage than others.

 

This happened to me on my F20 1 series, but only when the rear window heating element was turned on. The fault was a break in the element which apparently is also used as the radio antenna. This was evident on a frosty morning as part of the window wouldn't clear.

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Good shout above on the auto switching between DAB and FM. Other than that I would say with mine I don't notice any volume change even through the speed dependent volume is on - the change is obviously very subtle. So if its not the DAB/FM think I would suggest there is a fault.

 

My brother and I both have M140s and live rurally. Both found this happens when you go into and out of DAB reception.