[Repeater-Builder] Strang new noise in repeater

[NORM KNAPP]

Got a VHF repeater in my area. In the last several weeks it has started making 
a noise like that of a steam whistle. It doesn't do it all the time. It seems 
random. I have not been able to make any connections yet with regular 
occurances. It doesn't do it until someone talks through the repeater and the 
unkeys. The noise keeps the repeater up for a couple of seconds to neary a 
minute. The repeater is a 100watt MTR2000. I don't remember the duplexer 
type/model/brand. It doesn't seem to do it during the day, it seems more 
prevelant in the evening and mornings. 
Any ideas?
N5NPO
Norman Knapp