It is the sub for sure. I dont know if the room can make the sub make the resonance, but it certainty comes from the membran on the sub and it looks like it is in perfect condition.  

As suggested, is it the BL2 or your room, I'm going to immediately assume it is the room.

I had a BL2 and because of its strong sub output (I would hope so) I recall the room responded at something like 80 and 40hz. Even today with BL9 there's no getting around acoustics. Regardless of make, the physical placement of subs makes a world of difference. Other than physical changes there's no way to really 'filter' that out. You can certainly try but that only sort of reduces the true issue and you mess with the original music.

The BL2 is a sealed and competent design so resonates are mostly avoided by such.

If you have the ability to run a slow sine sweep at decent volume from say 100-10hz it is very obvious where the room resonates.

Hey David - I think (unfortunately) you are quite right. Though it seems strange what acoustics can do so when It sounds like it is the membrane itself. The sub is very solid. I can see some complaints about rumbling from this sub, but this is more like specific low frequencies that vibrate more than the content of the audio file. Maybe this sub is difficult to handle i smaller rooms and the system works better without. 

Get yourself a BL11 (or two) instead.

MM

Hey Millemissen

I thought about the Beolab 11 but the dealer said that it was not good - and it was in a B&O shop.

But the membrans and the cabinet in the 11 are bigger that in the 3´s so it must have som positive effect. Have you tried the 11 wih the 3´s ?

One of the ways to figure out where to put your sub in the room is this. Put the subwoofer where your listening position is, then play music with bass and walk around until you find the spot in your room where the bass sounds best to you, then move your sub there. Might not be a good place (aka middle of the floor) or still might not solve the problem but it's something to try.