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1999 F-150 whistling sound

I have a 1999 F-150 V-8 4.6 liter. I noticed a whistling sound coming from the back of the truck yesterday. I thought it was just when I put my breaks on, but it continues while driving. It does not make the sound all of the time, but I have been noticing it yesterday and today. Any idea what this is? We are leaving for Disney World from Ohio on Saturday, and will be pulling a camper with this truck. Any suggestions?

"Any suggestions?"
Yes, pick a cooler time of the year to go to Disney.

Is is really whistling, like an airflow type of sound or is it more of a high pitched grinding/rubbing sort of sound?
Does it do it only while you are moving, or will it do it if you rev the engine while still?
It could be: and exhaust leak, airflow around a loose trim piece or body panel, a brake issue, wheel bearing (usually a low pitched hum though), and so on.
Could you record the sound and post a clip of it?

When was the last time the rear differential was serviced?

I’d love to go in a cooler time of year, but we have to go between kids sports and school starting.
It sounds like the high pitch when the breaks need replaced, but I hear the sound after I let off of the breaks. I changed the front pads and rotors last year, and I had the rear drums checked in the spring. All were good then.
I initially was thinking the breaks, but I hear the sound on and off. It is not constant, but it seems to be there more when I use the breaks. I don’t know that I could record it, because it is so intermediate.

Can you change the sound by lightly applying the brakes?

I don’t think so. It seems to start when applying the breaks, but then continues. I did not notice it at all while driving to work today.

it has never been serviced

Well whether or not the differential is making your noise, you should have that done. 11 years and probably north of six figure miles is too long.

Could this be the U Joint and how expensive is that to repair? It was making the noise again tonight when driving home.