Dave,
Typically the built in mics in camcorders are notorious for picking up tape
motor noise, and other mechanical vibrations and recording them along with
ambient audio for posterity. Your problem described is sorta 2 fold:
"rumble" is low frequency (theoretically filtered out by a HPF = High Pass
Filter), but "noise" is
either broadband frequencies (wide spectrum) or relatively high freq. band
"hiss",
so a HPF wouldn't block that and any filter that attenuated the bradband
"noise" would most likely also attenuate your "signal" and thus not improve
your S/N ratio. If the "hiss from a movie" was tape hiss from an analog tape
source (e.g. a VHS tape movie) you can not clean it up and make it better
than the source.
Thus GIGO = Garbage In, Garbage Out. Once the signal and noise are combined
on the same audio track(s) they exist as 1 analog sound (or digital data
stream if encoded after the mix).
In case you're wondering, the way the nifty noise cancelling headphones work,
basically it never lets the background noise source get combined with the
program signal audio feed. It does that by collecting the ambient background
"noise" through an external mic or transducer and feeding that audio line
through 2 inputs of a "differential amp" chip, one of which is the reverse
polarized (mirror opposite) of the normal input, and the chips output is the
"difference" of the 2 inputs, which is zero level no noise. That then is
mixed with the true source program audio "signal" and output to the
headphones. So the noise doesn't get into the program audio, only the
preprocessed "zero summed" cancellation signal
gets mixed with the desired audio source.
However, analog tape noise added to the analog playback audio circuitry
noise is an inseparable noisy program.
But the 1 thing you can do, to return to the first HPF aspect, is filter out
the low rumble by inserting some "rumble filter" inline between the noisy
source audio and the input of the recorder/digitizer/whatever. My Mackie
mini mixer has on each of it's mic input channels, a HPF mic rumble cutout
filter button that chops out low ("rumble") frequencies, below about 150 Hz.
If you could find some filtering device like that it would need to accept
LINE LEVEL inputs and output LINE LEVEL to your
recorder/processor/digitizer/whatever, rather than unamplified MIC level
signal, as in my Mackie mixer example.
Good luck!