Oil-fired water heater question

Just bought a fairly new house with an oil-fired water heater (standalone, not something that hangs off the heating system), and it's doing something that seems strange.
About a minute after it starts firing, and continuing until maybe a minute after it shuts off, there's a kind of gurgling sound coming from the tank that really sounds like the water is boiling. This is accompanied by fluctuations in the pressure of the water which would also make sense if it's boiling.
My question is, is this normal for an oil-fired water heater? My worry is that somebody stuck a too-large nozzle in the burner in an attempt to get more hot water and now it's generating too much heat. The thermostat is set at the "normal" setting - 120 degrees.
Thanks!
Eric Law MA
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set
I understand sediment collecting in the bottom of the tank will produce these symptoms. The cure is to flush the tank through the spigot near the bottom. I haven't had the problem in many years. I believe the manual that comes with most water heaters recommends periodic flushing. --SJF
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