Sounds like maybe a chunk of something is getting caught up under the blade...? Maybe increase speed for just a few seconds to break up the veges.
I find with with the stock since it's pretty thick I need to give it a stir or two during cooking so it doesnt catch and burn on the bottom of the jug, and sometimes increase the speed to 2 just to make sure it's moving well.
When you aren't cooking anything, take the blades out and make sure around the seal is really clean - that's what needed a clean when I had some odd noises.
Good luck!