Rinnai loud exhaust on startup

[Dielectricunion]

I installed an Rinnai gas tankless for a friend of mine last year. He said when they turn on the shower every morning, it lets out a loud deep rumble from the exhaust (PVC). He described it as sounding like a didgeridoo. I did not check the gas pressure when I installed it but it is hard piped in 3/4" and not far from the gas meter. I can't recall the exact model number but I think it was a 8-ish GPM. Apparently, everything else is working as it should.

Any idea what would cause this?

 

[Fitter30]

Was the burner was set up with a flue analyzer? If not it might not be burning correctly.