I recently 'answered' to another user's question to corroborate their circumstances based on similar details/experience I had. My supplemental details are added to their post here, but to restate them:
I had had an AC power issue, so replaced the motherboard (before the power issue everything worked fine). After replacing the mobo, I resituated CPU, memory (using all the possible configurations of modules), and power adapter plug and powered it on. For a few seconds (say 6-8), the computer comes on with solid LED light patterns and the fan running, then it turns off and reboots in an indefinite looping cycle with no noises.
I am hopeful someone at HP can help me understand why there's no noise, I even replaced the memory with new equivalent DDR3 sticks. I appreciate your product & consumer support!! thanks