O\D sounds..TR4

[Bob <rlmcneit@cpis.net>]

I took my TR4 to Charlotte this weekend.  It was the first time to
really try out the O\D on a long trip since I installed it.  It seemed
to make a deep hollow sound(rhythmical) when engaged.  I thought maybe
it was the exhaust touching frame or something, but without O\D engaged
the deep roar is gone.  For all I know this is a normal sound, since
I've not been lucky enough to have an O\D before.  Is it?
                                Bob Mc