Weird noise, anyone

[eberlestock]

I took off on a huntin trip a week ago and only got 20km down the highway before my truck made a weird noise. Its hard to describe. It sounded like something trying to seize up and with a vaccume type sound. It made this noise for about 2 seconds the first time and probably less than a second many other times. The sound is the same whether you are going 10kmh or 100kmh. I thought maybe a wheel bearing was going out, but the are tight and running cool. The drive belt looks good. Driveshafts are good. Maybe the idler pulley? The vaccume sound has me puzzled. Has anyone ever had this. My truck is a recreational unit only. When I am in it I am going to have fun and I dont need it to break down. Please pass on any ideas, thanks.