A length of flexible pipe - something like fish tank pump tubing? - long enough that you can stick one end into your ear and then move the other end around the area where you think the noise is coming from. Take great care not to get it tangled in the drive belts pulleys and be ready to let it go in a hurry if you do. I've done this for years and it's a very good way of narrowing down to a specific component. Some people like to use a long screwdriver where you rest the "pointy" end on the suspect component and hold the handle against your ear. This works particularly well for identifying the likes of alternator/water pump/idler bearings and other "weird" mechanical noises but in this instance I think I'd try the length of pipe first. This is a common fault finding technique for mechanics but if you're not completely at ease working on running engines then be very careful what you're doing - lots of hot and rotating parts to hurt the unwary.