subwoofer buzz - how to eliminate it

@grislybutter I initially missed your comment that it's downward firing.  If it were me, I would also use a subwoofer platform with the Nobsound feet on top of it to get the sub raised further above floor level. This combo together will give you cleaner sounding bass and should probably help with the rattling or whatever it is.

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.