DAB, being digital, has the *potential* for delivering excellent interference-free sound of CD quality. But, as with so much digital technology, everything is in the implementation - because DAB and digital TV use lossy compression, broadcasters have the choice over how far they turn down the quality in order to fit more stations into the resource that they have paid for. And (IMHO) they sometimes turn it down too far.
In the case of DAB, there is also the need to supply a good signal. If the signal deteriorates a bit below perfect, there will probably be no audible effect (whereas for analogue forms, especially those involving AM, you might get more hiss). For digital, if you decrease the signal level further (especially if there is a neighbouring signal that becomes stronger relative to the one you want) you will suddenly reach a point (the "digital cliff") at which reception falls off suddenly.
I imagine it is the sudden fall-off which causes the "bubbling mud" noise. I've never heard it because the only times I've used a DAB radio in a car, reception has been good.
FM reception near me is very poor. It may be partly that my car radio aerial is not as good as it was (cable fault or aerial pre-amp fault) but BBC stations towards the low end of the FM band (ie R4, R3 and R4) are very fluttery whereas Classic FM and BBC/commercial local stations in the middle and the top of the band are fine.
At first I thought it was RDS that was not switching to a stronger signal, but when I rescan (having stopped the car!) there *is* no stronger signal. Where I live (east of York) the strongest BBC FM is from all the way from Holme Moss, rather than from my local TV transmitter, Bilsdale, because that has a polar diagram than aims the signal northwards.
This is something that I've only noticed for the past few months, so it looks like a fault (presumably at my end) that has only recently developed.