Heatsink options? Please help.

Lately I've been noticing some weird suddenly heavy activity of my CPU heatsink. It suddenly starts going wild, and then turning down again. And on another topic I made on tomshardware, they recommended that I got a new heatsink. (Was a topic about temperatures)
So what I'm asking for is some help choosing the right heatsink, that fits my CPU and motherboard. If you have any recommendations, please type them in!  

CPU: AMD Phenom 2 x4 965
Current heatsink: Stock that came with the CPU
Motherboard (info from SIW): Microstar 770-C45
Height from CPU to my case: 17cm (This is the maximum height the heatsink can have)
Exact case: NXZT Beta (older version, the power supply is in the top)