Very loud cracking/creaking sound on the bike? On bumps a very loud click sound comes from somewhere on my bike. 

When I have my weight on the seat and there is a bumpy road, it's a scary loud noise since 2 days. It developed slowly since one month until it has become a scary sounding fraying cable / floorboard sound.

I touch my hand on the top tube and I can feel little hammer hits inside the horizontal top tube when i press back/down on the seat post, as if the top tube is generating a stretch stress sound.  

Front forks and head tube and bearings seem perfectly sound, checked them. The crank is fine. The back is fine.

Has anyone encountered something similar?
How can german Aluminium MTB make scary crack sounds? 
What zone of a bike can do that?
It was the rear suspension bearings, I placed WD40 on them and it was fixed. It seems that they were stiff enough to be jumping around same as a stiffly creaky door rather than actually turning.