I have a 12 year old A.O. Smith that makes a lot of noise when heating.
Smith has a very detailed document on how to delime but it does not seem to be a very common procedure since I can't find any stores which carry a delime kit. A search turns up advice on using CLR or vinegar.
Is this worth a try?
I searched here and found another thread that seemed to suggest living with it until the unit dies or replace it.