Dell Dimension 8400 Start up Fan Noise

Hello i have a dell dimension 8400 - when i go to turn the desktop on it starts up normal for the first 2-3 seconds then the fans start up very loudly. i have cleaned the fans, and anything else i could see that had any kind of dust/dirt build up. checked the wire connections. i cannot see any kind of damage to anything inside, nothing burnt or melted. after the fans kick in the computer does not boot. However if i go the fan near the heat sink and press inward on it the pc boots up fine and works normally. Is this a fan issue or a cpu issue? or something else?