Why is there clanking or grinding noise when I'm driving?

is it my engine? or my catalytic converter? I had the "check" engine sign on for about 3-4 years now, never bothered with it as I know it was my catalytic converter. I was told it's not all that important, but I do know it needs to replaced someday. I guess it's time? My car has about 167,000 miles on it, it is a 2001 Toyota Celica GT. I had my oil changed recently, but when I checked my oil level, it was almost empty. It was about 1000 miles early before the 3000 miles that was suggested for my oil to run out or changed, which was weird. I don't get it. Did the mechanic at Midas try to rip me off by putting less oil so I'd have problems? Also, the mechanic at Midas never mentioned that my battery needed replacing, so I got stuck with a dead car. I already changed my battery so I'm good now. My battery was about 2 years old so I should have known better since my dashboard lights was flickering and starting the car seemed slower than usual. I use a 5w30 engine oil. Some guy told me to use a 20w50 engine oil. Is this ok?