The interview audio I recorded today has HVAC noise mixed in. How can I save a version on-site?

I was filming a client interview today, and there was noticeable low-frequency HVAC noise in the room, which I could already hear in my headphones during the test recording. Since the AC couldn't be turned off at the venue, instead of just finishing the shoot and gambling on post-production noise reduction, I moved to a position further away from the vent, secured the lav mic more firmly, set up…