What could be wrong if my car looses it's acceleration power?

I drive a 2008 Toyota Yaris Sedan 1.3. It's a 2nd hand car & I don't have the service history. The car always looses momentum when I reach a steep hill. It's as if I'm standing still & all the other cars are driving past me. I have to change down the gear to atleast 2 in order to get it moving properly. The exhaust pipe also makes alot of noise when I start the car or when I'm driving. It sounds like a blocked chest.