The message
from John Rumm contains these words:

Any other explanations you can think of?

Change of speed of sound in the fluid. That should affect the resonance
much as breathing helium affects the pitch of your voice.

Rotation of the fluid in the container. Particularly if the fluid is
rotating faster in the middle the propgation may be more spiral than
radial, so the path is longer giving a longer wavelength. As it slows
down the path shortens and the pitch rises. I really liked this idea
until I tried it. Milk and honey with nutmeg on top to show the flow and
the pitch continued to rise significantly long after any rotation
ceased. Shame 'cos it was quite plausible! It may still play a part in
the early stages though. I need to try it with a stable fluid rather
than a freshly hot one.

What would really help would be having a name for the phenomenon to make
it easier to find other research.

--
Skipweasel
Pay no attention to that man behind the curtain.