[plt-scheme] Debugging non-scheme-heap memory leaks

I have a program that basically does a bunch of computation and reads
and writes data to binary files.  I believe I've eliminated anything
involving FFI, but I'm still finding a memory leak.

Running with '-W debug' shows that the scheme heap doesn't grow, but
the programs memory usage according to top continues to gradually grow
as the program runs.

Any ideas on how to go about debugging this?  I'm using 4.2.3 with 3m.

Thanks,
David