Why is my vacuum making a really loud sucking noise?

After about 20-30 seconds of vacuuming it starts to make a really loud sucking noise. Yes, I'm sure it's not just the regular vacuum sound. It almost sounds "sputtery" like it's not getting enough consistent suction. I've cleaned out the rotating brush area and used my dryer vent cleaner as far as I could through the hose. It's a Kenmore Progressive upright bagless 12 amps. Model number is 116 36933502 Thanks so much!