Boiling sound?

[MatBirch]

I have Goodman 13seer heat pump, new in 2015. I’ve been in the house now for 8 months. Now that we’re full tilt into the summer A/C season, I’ve been noticing that in one room I hear a boiling sound coming from the return air duct. It’s a very old house and uses the joist cavities as the duct, so in this room, it a pretty straight shot, maybe 20’ away.
Any ideas?

 

[Wrench97]

I'd be listening to/looking at air handler to see if there is a build up of condensate or a blower issue.