I just changed my engine oil (Castrol 10-30) and manual trans oil (Mercon III) in my '91 w/ Mazda 5 sp. trans. Then took her out for a test drive. Now I'm getting a strange noise when accelerating at higher speeds, sort of like small peas being shaken in a tin can. I just went to the garbage to double check the oil grade and all is to specs. including fluid levels. Lifter problem or what? Or just a coincidence?

id say coincidence....changing fluids should make something sound broken.....

Sounds like gas ping:)