water heater - rumbles loudly

I have a gas fired water heater (American flameguard) that is 4.5 years old and I also have a water softener. After the water was installed for six months, the unit began to rumble loudly when a lot of hot water is used for a shower. I am in the room directly above the water heater and you can hear it thump and rumble 4 or 5 times during a 10 minute shower. The plumber was not much help so I decided to order an anode rod. The plumber put it in and the rod did not look eaten but was encrusted with a quarter inch of white stuff. The noise stopped but then returned after six months or so. So I have been changing the rod every time but I am still not sure what is happening. I recently flushed the tank and nothing came out. I have not tried draining it however, which I will probably do soon. But I am looking for any suggestions people might have.

Another symptom that I see is the upstairs bath faucet expels a burst of gas/air out of the hot water side after its running a few seconds, this problem disappears when the anode rod is changed and the thumping goes away, the first floor kitchen sink and bath do not have the problem, any ideas on this would also be appreciated.

There's a pile of mineral deposits on the bottom of the tank that trap air bubbles when the burner comes on.

If the softener was there when the WH was replaced, something's wrong, you shouldn't have all those deposits. Doesn't sound like it is working properly. Could be programmed wrong, or it needs maintenance or repair. There should be few if any deposits in the WH if using softened water.

heater

"Nodes" have built up on the inside of the heater's flue passage. They fill with water, and get heated very hot, actually to a superheated temperature. When you open a faucet the pressure drops a bit and the superheated water is forced out of its node and condenses with a miniexplosion which is the rumble you hear. Eventually the node will either destruct, or the constant erosion of the explosions will cause a leak in the tank. IF it happens within 6 years, which would be uncommon with a gas heater, it will be replaced. The tank will be replaced, but not reinstalled.

Node??

Thanks for all who replied,

HJ, Why would replacing the anode rod every 6 months or so resolve the thumping for a time? The rumbling and faucet problem do not occur at the same time.

I am going to guess it has something to do with reducing the water pressure to 0.

node not anode...

"Main Entry: node
Pronunciation: \ˈnōd\
Function: noun
Etymology: Middle English, from Latin nodus knot, node; akin to Middle Irish naidm bond
Date: 15th century
1 a: a pathological swelling or enlargement (as of a rheumatic joint) b: a discrete mass of one kind of tissue enclosed in tissue of a different kind
2: an entangling complication (as in a drama) : predicament
3: either of the two points where the orbit of a planet or comet intersects the ecliptic ; also : either of the points at which the orbit of an earth satellite crosses the plane of the equator
4 a: a point, line, or surface of a vibrating body or system that is free or relatively free from vibratory motion b: a point at which a wave has an amplitude of zero
5 a: a point at which subsidiary parts originate or center b: a point on a stem at which a leaf or leaves are inserted c: a point at which a curve intersects itself in such a manner that the branches have different tangents d: vertex 1b"

Basically a closed in pocket where the water gets heated then when the pressure drops during use it converts to steam. 1700:1 expansion hmmmmm.