I use a Seachem Matrix as bio in my FX5 and this pebble was the size of Eheim coco puff (substrat pro).
UPDATE -----------
Ever since I took out the pebble the noise has been reduced drastically. It still gives a humming sound and is not dead silent as the Eheims are in my experience. Then again 900 G/h motor output vs 330 G/h.
I cleaned out the filter opened the impeller. I found a small pebble in there that was causing all the noise and vibration. Problem Solved. Thanks for all the replies.
Ps. I Love this filter. Water is crystal clear since I installed it and no more noise.