Hiss can be caused by a dry joint or a component beginning to fail. Let me tell a story against myself.
In the 1980s we bought an A&R Cambridge A60 from Reading Hifi. I had some sensitive speakers – anyone remember RAM 150s? After a few weeks I became aware of a hiss from one channel. I took the amplifier back twice and they could not find anything wrong. On the third occasion they demonstrated using less sensitive speakers – no hiss.
Despondently I went about trying to cure the fault myself. With cover off I prodded various components. Something metallic somehow fell on the circuit board. There was a phut and splutters as various transistors combusted in front of my eyes. I went to bed telling my wife “I’ve just blown up our new hifi amplifier” (A phrase which has been repeatedly echoed over the years when I am about to attempt technical audacity.)
I wrote to A & R – they were very good, supplying a fistful of transistors and diodes very reasonably. To save face I told them I was a repair man trying to make good the errors of an unknown idiot. I won their sympathy. I worked for three weekends in the spare bedroom. My wife however put a sign on the door “dog-house”. She has a cruel sense of humour at times.
In the process of rebuilding the amplifier I found offending dry joint on a resistance. I was also able to upgrade a few passive components at the same time. The A60 is still in daily use: it is a great little amp.