Need Help Diagnosing Air Handler Noise/Problem

I'm having a problem diagnosing a situation with my Air handler inside my condo. Its a PF4M NA025 Payne model and whenever the AC turns on it seems like the blowing speed is fine for a minute or two, then slows down and I can hear a noise coming from the air handler. almost like a vibrating/shaky/humming noise. The air blowing out when slowing down is still cold, but nowhere near the usual blowing speed, barely a light breeze. I'm wondering if this is common or if anyone has any ideas on where I should begin to look to find a problem. Thanks