My mechanic does not know the answer
 

I have had a problem with a harmonic in the rear engine area.
I am not saying in the engine itself, because when it occurs, all mp and rpm remain perfect.
This harmonic has occured mostly on landing and just after putting down flaps and gear.
It seems to occur while the gear is in transition to the down position and right around the time I move the props forward.
The plane shakes and seems to go away with a reductiion of power, and it lasts only the time it takes to turn from downwind to final.
It also occured with my mechanichis afternoon. On this occasion, I was departing, had not reduce the props and had just cycled the gear (500 aGL). The harmonic started, this time I reduced the props and it went away.
I am open to all ideas, from Props to doors.
I need to get this figured out quick, as I am leaving for OSH in 10 days.
Thanks
And if you want to call me to discuss, my cell is 410-916-0039, I will call you back and pay the charges.

Rick