Funny whistle noise.

I've had my truck for a little under a year now, but I've noticed a wierd noise from it lately. It is a low pitched whistle that starts up about a minute and a half after the engine is warmed. Sometimes the sound is worse when I rev the engine a little, sometimes it goes away when reved. Also, sometimes the truck doesn't make noise at all when running. This is all at idle by the way. The noise is very hard to hear where it comes from. Sometimes it sounds like the alternator area, sometimes it sounds like the fuel pump area. It's really hard to determine, but the sound can be pretty loud. Any ideas?

May be the fan shrowd contacting the fan. Doesn't really make a whistling sound (IMO), but different people describe sounds differently. When the fan clutch heats up, it may be sucking the shrowd inward. Make sure the radiator and condenser are clean.

Tensioner or idler pulley can make that noise. Try removing the belt to see if the noise goes away. Another option is use a long screw driver and hold it to your ear, then touch it (carefully) to different area to see where the noise is coming from.

//2000CAYukon

Thanks for the input. I know it's not the belt tensioner atleast since i just replaced it, and the noise sounded before I did the work. I'll check into the fan issue. It's definitely coming from around the fan area somewhere in the middle, sometimes it sounds squeaky too. The sound actually varies a bit on some days. Looks like it's time to do a little diagnostic work. Thanks again guys.

most wistle noises are actually vacuum leaks. you can eleminate everything with a pulley by running the engine with the belt removed. just don't do it for too long or the engine will overheat. pay close attention to all vacuum lines sometimes they rub on stuff or get burned. a can or carb cleaner of wd-40 a fast way of finding leaks. spray around the throttle body base gasket, lower intake gaskets and vacuum lines. it's fairly common for part of the throttle body base gasket to get sucked in. good luck

actually i put a holley economizer 4 barrel on my truck a while back. i ended up putting the larger one back on but i noticed when i had the economizer on that it would frequently make a whisling noise when the butterfly over the primaries was at a certain percentage of opening. also it only did it in about a 200 to 500 rpm range and would go away once airflow increased at high throttle amounts. at really slow idles it would also go away. it was kinda annoying but i guess u could just turn the radio on and not hear it lol. i decided to put the old holley 600 vaccuum secondary carb back on that old 350. i did think i had a bad valve in the number one cylinder cause it would backfire a lot with the old carb on but the new carb made it fast as hell and it would not backfire at all. it was very confusing to me. however the part throttle on the new carb sucked as it sputtered if the throttle was anything other than idle or full throttle so i put the old one back on. as long as i dont put the pedal too far down it doesnt backfire, and it also helps a lot if its warm out for some reason. im gonna try that acetone stuff because obviously i need a new engine anyways since i have lots of oil burnoff and i think a crank bearing is ticking or something. anyways i think ill try it out this weekend and see if i can get the damn posi rear end to break loose maybe. ill get back on that one

The acetone won't make a huge difference on power. My truck would turn both tires over a bit (could hear them grind), it was probably just a horse or two short of what the chirp needed.