I suspect that you have 'foam rot" which is where the rubber connecting the paper speaker horn to the frame has split. You can with try repairing the speaker (there's a guy in Holland who supplies kits) or buy new ones from a B&O dealer - I think that I paid about £60 per speaker unit.

I repair these woofers.

They become hard and get cracked around the outerside of the roll.

I use the new rubber rings with more flexibility and more bass at the end.

But that's only one problem of the BL3500. More foam is behind the display, which can cause malfunctions with it's display or ML behaviour.

I've fixed several and/or modyfied them for centerspeaker using (the MCL connector)

You can remove the grills by sliding them to the left or right side.

At first sight you will not see the cracked ring. But by touching them, you can see and will hear the same cracking noise.

This is a common problem with BeoLab 6000 speakers as well. BeoLab 3500 are basically the same speakers cut in half. The problem with the dried out cracked foam happens after 10 years or so. Easily fixed.

The woofers have the same diameter, but are a bit different. 

The Beolab 6000 has 2 woofers and a tweeter where the Beolab 3500 has 1 woofer and a tweeter on both sides for each channel.

The woofers have rubber surroundings and get hard and are not made of foam.

Beobuddy wrote:

You can remove the grills by sliding them to the left or right side.

At first sight you will not see the cracked ring. But by touching them, you can see and will hear the same cracking noise.

As you already felt. The rings are very stiff. These aren't made of foam.

You can easily overlook a cracked ring. I would suggest to play some music when the grill is removed.

By touching the woofer you wil infuence the cracking noise.

If this doesn't work, then I suspect you have a leaking SMD capacitor around the audiochip. Also not visible, but noticable by the smell when you touch it with a tip from a soldering iron.

Hi, I also have a problem with the righthand speaker in my 3500. I removed the grill and noticed that part of the black foam around the edge of the speaker has fallen out. Also when I played music through the speaker with the grill off some more of the foam fell out on high volume. As I am in Australia do you supply repair kits for this unit and do you supply instructions on how to replace the foam/rubber.

Bop wrote:

As I am in Australia do you supply repair kits for this unit and do you supply instructions on how to replace the foam/rubber.

Welcome to the forum 🙂. Reading your post it sounds that you think you are communicating with B&O.

You are not. This is a forum for B&O enthusiasts but I'm sure someone could advice where to source material for re-foaming your speakers.

There are some really knowledgeable people on this forum 🙂