Still could be a cap. Perhaps one that has dried up and is arcing internally one it reaches a certain charge, hence the few seconds delay. I dont know how to test this myself.

If anyone else does I would be interested in knowing how. I have Musical Fidelity amplifyer that gives off a pop everyso often, and I am thinking that one of its power supply caps is causing it to do this. When I have some cash together I intend replacing them all anyway, The amp is 12 years old, so my guess is that the caps are due an overhal anyways. The sound I am sure is not as detailed as it used to be in the bass range. But I would be loathed to replace such a well loved piece of kit.