[racket] cpu usage of DrRacket

I see that after starting up, DrRacket constantly uses between 40 and 60 %
of my two CPUs, even with empty definitions window and before doing anything
by myself.
 
I use:
Welcome to DrRacket, version 5.2.0.6--2011-12-10(6fae172/a) [3m]
without debugging and without preserving stacktrace.
Language determined by #lang line.
Disabling online expansion does not decrease CPU usage.
 
What may be the reason for DrRacket to use so much CPU at moments I would
expect it to be idle?
Thanks, Jos
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