You may have an air pump(serpentine belt driven) and if so, it's likely above the alternator.
This device can cause alot of racket at first start up in the morning. It's supplying additional air to the exhaust. Compliments of Ralph Nader. The noise it generates usually kicks off about 2-3 mins after the car has been started. This may not be what you're experiencing...thought I'd mention it.
1991 300-SEL - Model 126
M103 - SOHC
James Madison - Oppressors can tyrannize only when they achieve a standing army, an enslaved press, and a disarmed populace.
Liberty Firearms Training - After years of trying to regulate California criminals with a never ending stream of gun control laws they have still not given up with the belief that if they make something a crime that criminals will miraculously start obeying the law.