Rick,
My experience has been that the slapping you hear is not from the IMF, but  
from the wires that are run through the front opening inside the mast. This is 
a  common problem on all sailboats. You have wires that go upo to the steaming 
 light and possibly a VHF antenna lead in there. There are two ways to 
eliminate  the noise that have been discussed on the list in the past.
The first is to use wire ties and attach them to the wires every foot or so  
as you pull them through the mast. This process requires you to remove the 
wires  and reinstall them.
The second solution and possibly the easiest is to stuff the tube with foam  
pipe insulation. If memory serves me correctly, you need 2 or 3, four foot  
sections for the lower part (below the spreaders) and another section  for  
above the spreaders inserted from above.
Both solutions require your dropping the mast and removing the top plate on  
the mast. 
My foam insulation has been in my mast for about seven years and I haven't  
had any problems or sound with it.
Hope this helps.
 
Rummy