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There are two other possibilities you ought to check:

1. The noise might be propagating through the wall. Depending on
construction, some walls are quite capable of transmitting sound. If
that turns out to be the case, just a bit of foam or something where the
vent passes through the wall might fix the problem.

2. If something is indeed resonating, changing its size (and thus its
resonant frequency) might help.

Yes, that's definitely something I've thought of. Maybe as a multi-
pronged approach. Wouldn't be too difficult to enlarge the existing
hole/vent pipe. Thanks for the comment!