Didn't find this issue in a search, soooo:
Zone 2 (rear, call it Z2) radiators were blowing cold air while fronts, Z1, were blowing hot. Wife not happy
(wants full heat in Quartzite next week), so gotta fix it instead of put it on official ignore list.
Set Z2 for 90 degrees on the Dometic thermostat, & removed sheet metal cover on burner side of AquaHot 450 (same unit as the HydroHot 450 in earlier coaches, they just started painting it white & calling Aqua instead of Hydro). There are two 'circulation' pumps that have a clear plastic housing, one for Z1, other for Z2. These push hot boiler fluid thru the zone's radiator loop. Pump for Z2 (call it P2) was spinning but making a buzzing noise. Set Z1 for 90 degrees, P1 spun, no noise to speak of. Tube up from P1 to Z1 in coach was warm to touch immediately, tube from P2 was still cool, so not circulating fluid.
I tapped lightly on the in-line brass check valve just behind P2 (there is another for P1, presumably so you don't drain coach radiators if you replace a circ pump?), and instantly buzzing noise from P2 died (continued spinning), and tube warmed up. Checked inside & good heat to Z2.
Conclusion- check valve stuck, causing back pressure on P2 impeller making it buzz, but only a light rap on valve needed to open it up.
Fix- next to free, happy wife- priceless.
See you in Quarzite