Maybe a bad clutch on the a/c compressor!? Is it like a squeek - thud? Could be a glazed or loose belt too...or a bad a/c pulley...
I put my $0.02 in..
1988 Mazda 323 GTX; 1 of 1,243 Imported
AEM CAI, VJ11 Turbo @ 20psi, 2.5" IC Pipes,
Corksport IC, CFDF Clutch, 2.5" Turboback, Konis,
Suspension Tech Springs, 22mm Rear Sway, + More.