sounds like nocking/vibration on idle and squeeling/screeching issue

so i have a few random issues with my XJ. The car runs fine, idles properly, and doesn't misfire. When idling like when i first turn on the car there seems to be a slight nocking/vibration coming from under the floorboard. My mechanic hasnt looked at it but says it could be the cat converter or something is just hitting against each other under the car. Another thing is this started recently a couple of times, but i'll be idling and a loud squeel/screeching will suddenly appear. It sounds freaky so everytime i just turn the car off then back on and its away. I dont know if its the belt (i've replaced the belt and idler pulley couple weeks ago). Any comments to what to check or what my problems might be would be great.

Thanks in advance.