I have a 2006 Volvo S40 @ 108,000 miles....started hearing a high pitch sound about a month ago. You hear it after driving for several miles, mostly from the rear of the car, does not change whether or not you're idle, and gets louder the longer/farther you drive. Mechanic says it's the fuel injector and it needs to be replaced. Is that accurate?

 

This could be the oil trap which is covered under a recall. Also it could be, unlikely, the power steering pump. Does the pitch change when you turn the wheel?

 

Rear of the car? I think your mechanic might have meant the fuel pump which is located in the fuel tank. (The 5 fuel injectors are on the engine in the front. They make clicking noises.) Once the noise is good and loud try removing the fuel cap (leave engine running) and listening for the noise to be coming out of the tank. If it's the fuel pump then you can hear it best there.

There is also an air pump under there for the emissions system. If it's not the fuel pump you need to get under the car and try to pinpoint the noise.