Hello Hans,
There is a detailed article in the Super Seal Manual ( see the main menu / about the PSSA/regalia and manuals ) by Dick Landon ref 91/4 where he investigates resonance on a Parker 27.On his boat it occurred particularly at 5.25 kts.Briefly,he finds that using rubber spacers on the keel guides instead of steel washers was partially effective as was improving the leading edge and the trailing edge of the keel.He also considered that stiffening the box would further help but did not pursue that.
I suggest that you should read the full article which is complex.The manual costs £10 to members.It is worth checking that this particular article is still bound within the current manual version,which has lots of useful information.
Peter
PS We do not have this issue on our Seal 27 #131 but do have a substantial full height hardwood strip roughly midway on each outer face of the box with no other obvious purpose (of course they are twin compression posts)so perhaps this was a works modification by John Baker or Bill Parker that was applied to later boats ? It would be fairly easy to add such strips ( if the decor allows) using Sikaflex 292 to bond the strips - if they are not already on your boat.