baja02

Hi everyone, I have a unique strange problem that 2 mercruiser service mechanics don't know the answer to and I'm tired of hearing,,(don't worrie about it)

I have 2 mx 6.2 mpi's in a 29 foot baja..When I take the boat out of the water the starboard drive drains whatever water is left in the drive...Now the port drive keeps dripping on land for a few days and when I put the boat back in the water it takes awhile for the water to come out of the exhaust pipes while the starboard motor the water comes out of the exhaust immediately...
Note...The port motor runs at 165 to 170 degrees..no overheating..idling down to 160 degrees..Complete new sea water pump thermostat, No water leaks inside the engine conpartment and the exhaust is running cool....But I hear a strange little whinning noise when turning off the engine..ignition keys off, no pumps running...I have a blue drain fitting at the top side of the thermost housing..After shutting off the engine, if i remove that drain fitting no water comes out but you hear a suction of air in..The starboard engine does the opposite water comes out...The mechanic said something is causing a vaccum situation and is draining part of the engine block back thru the sea water pump and out the drive while on land sitting....

any input would be appreciated