Squeal from engine

I have a 2006 Prius, which has 96,000 miles on it. It has just developed a high-pitched squeal that happens intermittently (so it seems). No squeal at high speeds, though. Fuel economy is a steady 53 mpg, which isn't bad for a 6 year old vehicle. What sort of issue is this? I just read a thread about the air intake system perhaps being cracked. I was thinking perhaps it's the engine belt squealing. It has never been replaced to my knowledge. Any advice?