Selgas
From the description you have given I would suggest that the gurgling sound you are hearing is sediment moving about in the base of the cylinder over the burner area. Possibly this could be removed by simply draining and flushing the cylinder out – Turn off the gas to the cylinder FIRST before you attempt this otherwise you will damage the cylinder with the heat build up for sure.
The next part of your question is easily answered as being expansion. This would be more noticable at times when little or no hot water has been drawn from the cylinder and is quite normal. At times of heavy drawoffs of hot water the cold supply pipe would feel cold as the water being pushed into the cylinder through the inlet pipes would not have had sufficient time to have been heated by the burner.
Hope this helps. 
Selgas Services Ltd
Craftsman Gasfitters, Plumbers, Electrical Service Technicians
