If there is a noise with no power to the TV...
1) If it has been stored in a cold environment and moved into your warm home, it is likely resulted in condensation. The high voltage circuits inside the set maintain a charge for a period even when switched off which is likely reacting with the condensation. Leave it in a warm room to dry out for a day or so.
2) Another possibility, although this usually results in a single, sudden loud cracking noise, is slight expansion movement of the heavy tube fixing against the plastic cabinet mounts.
Cobweb, December 2004