But since the noun is in the back then it is less important than the rest, so the frequency is getting emphasized instead of the what is being heard. I guess context is kinda missing here to make this sentence more acceptable. "I am travelling through this side of the country and i frequently hear rivers". Other contexts could be tinnitus, or well.. hearing things. When the what is more important then the context could be "the noise pollution is not that bad where i live and i can hear the rivers frequently"