Water heater is rumbling and I'm not sure if it needs replacing

[jacintocollier]

Our water heater has started making a rumbling sound, especially when it's heating up, and it's gotten louder over the past month. I've read that this can be sediment buildup, but I'm not sure if that's something that can be fixed or if it means the unit is nearing the end of its life. I want to get details about what a plumber would actually check before I call anyone out, since I'd rather understand what I'm paying for than just accept the first diagnosis. Has anyone dealt with the same noise and did flushing it actually solve the problem, or did you end up replacing the unit? Also, how do you know when it's time to replace rather than repair, because I don't want to throw money at a unit that's about to fail anyway.