Installing a smaller tank (as "JLNewc" said) gave me space for more foam and therfore reduce the vibration to the minimum.
I know 320cc is a very small tank for the 91 engine, (it just gives me 7-8 minutes max of flight). The idea was to find the solution and fix it. Now all I have to do is cut off some ply wood exactly behind the firewall to make the extra space I need to install a bigger tank with LOTS of foam around it.
I think it does make sense..............