Brand-X

So i have a 2004 Mazda 3 with about 45,000 miles on it. Back in Jan or Feb I went to get an oil change, but they were out of conventional oil, so I purchased synthetic blend. Ever since them I've been getting a rattling/ticking noise when I accelerate, and its annoying as hell. But it doesn't do it in 5th gear though. I don't know if something under the hood is loose and needs to be tightened, or if I just damaged my engine. Can I go back to conventional since it is a synthetic blend and not full synthetic. Oh, and my engine light came on twice since this happened.