subwoofer buzz - how to eliminate it

Also consider placing a 24" square paving slab on the floor. Use pads or spikes under slab. Then put sub on springs on the slab.

You might also consider moving sub to a less excitable location in the space.

@gladmo Or I could just turn it sideways, it was meant to do that, I am looking for a clean and simple fix

@noromance what's the WAF of the slab? I would have to submit designs and as-builts for approval

Convert to front firing. 

@grislybutter Turning sideways may or may not help. You'd have to try it. Raising the height using a platform helps whether it's front or downward firing. Personally, I wouldn't bother with the slab, even though it's probably a good solution. I would just put the Nobsound springs on top of an Auralex Subdude platform for excellent isolation and a little height, and also play with placement if you are so inclined.

Well, it could be that the sub is particularly badly placed for that particular frequency.  First just try moving it more into the room as an experiment, see if it not only stops shaking but also just sounds better. :)