M_Boats: Centre Board Slop

Hello All, Am a new M-15 owner, very pleased with my boat built in 1981 and sporting the original sails no.121. Have had a dozen outings in the last month in all weathers. The centre board in some wave conditions seems to knock against the trunk, I think in a lateral motion since the pendant is properly set,. Is this a normal occurrence with the M- 15 ? Also I tend to accumulate water in the deep spot behind the trunk and have to sponge out every few days. Any idea why.? Having owned DS- 16, Siren 17’s and Compac 16’s ,none compares with Jerry Montgomery’s little marvel. Each sailing session is a treat. RTH.