When I make Horlicks or hot chocolate, if I let it stand for a few minutes and then stir it up, the note made by the spoon hitting the sides of the mug starts high, then goes lower.
Why is this? It's keeping me awake at night which kind of defeats the object of the exercise.
Ever thought doing some charity work with all that spare time you seem to have? 
The fluid level is dropping as the vortex slows down so more mug is exposed.
That's my theory.
Edit or the other way round....I dunno why you asking me....is it alright when you make Horlicks?
Jamie, when I finally crack this mystery, I won't have any spare time. I've an awful lot of sleep to catch up on!
i think you have bigger things to worry about in that it's a sorry state when you are drinking hot, milky drinks before bedtime.
Maybe the density of the fluid increases as all the stuff that had settled out goes back into suspension, so the sound frequency will drop. You tried stirring it on a conveyor belt?
Actually youve just reminded me of a minor rant I have every morning.
May I Hi-jack your post?
The ceramic's in the cup are under stress with hot liquids, tapping/hitting the cup releases some of the stress, resulting in a lower tone each time you hit/tap it.
Oldgit... No. You mayn't.
The horlicks / hot chocolate that's settled to the bottom is now in suspension, changing the density of the fluid. This will effect the transmission of sound in the liquid...
Centrifugal force raises the fluid level in contact with the cup, changing the resonant frequency at which the glass vibrates and generates sound in rebound from tap.
Oh 
Do you share breakfast with a noisy stirrer? If so, I don't want to know about it. I've got enough troubles of my own without you burdening me with a noisy stirrer. Soz.
No I was going to discuss my new plug holes 
Like I say, enough troubles of me own. Let's solve this problem first. I'm liking Mogrim's idea best. Do you have any formal qualifications to back up this assertion?
Degree in Aeronautics, lots of fluid dynamics and thermodynamics.
Now working in IT, of course 
I've got a new plug hole related problem that I'd like to discuss too. May I hijack your thread shibbo?
Though jockhaggis's suggestion makes sense, too. Might be a bit of both, of course. Does it change tone if you use pure water?
Oi - mogrim stole my idea! Degree in mathematics, now designing gardens of course!
Sorry, thepurist. Didn't notice your reply, promise! Great minds and all that!
I think the molgrips has the primary controlling factor - the suspension will alter sound transmission in the fluid. Add to the fact that as you stir the liquid raises up the glass removing the higher frequency portion of the sound by damping the mug wall vibrations a little.
Don't forget the fascinating difference in sound that pouring hot water makes compared to cold, due to the fairly large difference in viscosity - thats another interesting thing to keep you awake at night!
You're forgiven (and probably better qualified than me!)
Yeah and I think jockhaggis just sexed up my theory?
They did this in New Scientist. It does it with instant coffee too but only if it's black apparently. Doesn't do it with water either which blows the centrifugal force thing out of the water, so to speak.
I'd be interested to see that article.
It was in their last word/notes and queries bit, where you write in with questions - forget what it's called. I think they are searchable tho.
'ovaltine' for me.
I found it, but it was answered by joe-bloggs, and concluded the change in noise was due to bubbles incorporated into the liquid had changed its stiffness. I'm not sure whether I trust the answer.
http://www.newscientist.com/article/mg14119106.700-letters-bubbling-up.html
Sorry guys, just been building a conveyor belt out of a pair of redundant Rubber Queens. I'll report back...
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