If it is a resonance you should be able to affect it by damping the pipe.
Try few hose clips clamped around the pipe to add mass in case it is just the pipe length and method of attachment that is the cause.
Or try some rubber strips under the mounting bracket.
Or cable tie some lead strips to it (lead roof flashing)
It is plausible that the modulation of hp regulator is the root cause but is should be possible to locate exactly where the noise is being produced and damp it.
Ron
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