Strange Doppler Effect In Fsx

Hey,I was experimenting with ai and user aircraft engine sounds when I discovered that when the aircraft passes by and away from the viewpoint, engine sound freq tends to increase rather than reduce. The fsx effect fits correct only to rumbling sound of backward moving exhaust gases but not jet whine. The whole thing gives impression of increasing rpm when it actually is constant. Dont know if it is true for ground traffic and other sound generating objects in fsx.Is there any way to force normal doppler effects for selected wav files? use of flags, etc?