the "buffeting" is more an audible phenomenon-the airplane sounds like a tractor !! The noise starts instantaneously and abates with reducing airpseed below about 120 kts It also disappears instantaneously when gear is selected down (thus the door moves away from the fuselage).
Dominic flew underneath in his Lancair and detected just a slight "bulge" in the left main gear door but could not appreciate the vibration visibly.

Gerard,

 

OK.  Here are some things you may wish to consider (assuming you do not have outback gear):

 

0.  Stop approaching Mach speeds.

 

1.  If the joint between the stub wing and the outboard wing is not filled, there is interesting air movement thru the wheel well.  You might want to seal that gap, even if just for a test flight, with tape. Some have been successful with vinyl electrical tape as a temporary measure.  Mine are more permanently sealed with white silicon.

 

2.  The middle door controls quite a bit of the fit, certainly where it meets the outer and inboard doors.  One adjustment is to bend the bracket tabs a bit to help the curve of the door match the inboard door better.  Another adjustment is to change the length of the standoff near the top of the middle door.  This adjustment can affect the fit dramatically at both the inner and outboard doors.  See what adding a thin or thick washer to the standoff does to the fit.

 

3.  Usually the fuselage lip that the closed doors rest against is flat - that is it does not account for the door material that was removed where the doors overlap when closed.  If the doors are not tight, that gap could allow air to enter there.  It is possible to build custom seals on the fuselage lip in that area out of RTV Silicon.  Whilst on jacks and after other door adjustments, apply a slug of silicon to the fuselage lip in the offending area.  Place a sheet of Saran Wrap over the silicon and retract the gear.  Allow to cure.  Open the gear, remove the Saran Wrap and Voila!  You may want to tape off fuselage areas that you don't want the excess silicon to stick on.  Also, wipe off any excess after the gear is retracted.  The excess on the inside can be cut off with an Exacto blade later.

 

4. Did you seal the push rod openings in the cockpit closeout rib?  That may also affect the airflow in the wheel well.

 

If you do have outback gear, the concepts may be the same but the adjustments may be different.

 

Note that a custom seal for the nose gear door can be made in the same way as described in 3.