I have very hard well water (iron, calcium, etc), and have never flushed my 8 year old water heater (40-gal, natural gas). Needless to say, there was audible snap, crackle, pop, and occasionally boom sounds.
Rather than shell out $$ for a new unit, I performed the following procedure last month with the help of an adventurous plumber:
1. drain tank
2. add two gallons of white vinegar
3. wait 12 hours (at about the 6 hour mark, I ignited the burner for a minute or so)
4. flush tank
5. repeat steps 1-4
It's been one month now, and still no noise!
Just thought I'd share...