My 2003 Odyseey has a intermittent high-frequency whin coming from the front. What is the cause?

It could be in need of a transmission fluid change. If the filter gets clogged the front pump could cavitate, or suck air, and in doing so will make a whining noise which can sound like a whine or a whistle. It could also be something unusual -- I had a 1989 Buick Century that made a rapping/vibrating sound, I ended up finding out this came from harmonic imbalances in the power steering fluid lines that ran along the firewall -- the fix was to replace the lines. I'd look at the transmission as a possible culprit first, have the fluid changed. Yet another possibility is a bearing in the alternator, power steering pump, etc. As the bearings go out on these accessories they can cause whining/grinding/rattling sounds, depending on how they are failing.

We have been experiencing a similar issue with our 2011 Odyssey and for a while I thought I was losing it, as no one else seemed able to hear it - until my 6 year old one day told me he did! Yay! Turns out for us it is the blower (heat and A/C) motor. Like your noise, ours is intermittent but gets worse the higher the fan is turned up. I figured it out one day when I turned everything in the car off to try and pinpoint the location, and everything stopped once I hit "Off" on the fan. My brother is a mechanic and told me certain Hondas are infamous for this issue (has a friend with a Civic that has gone through 3 replacement fan motors) and the sound generates from the bearing wearing out. Some days it's like spending an entire car ride at the dentist's office but since it's an expensive repair, I'm putting off replacing the motor for as long as I (we) can stand it. Give it a try, and I hope this helps.