Hello I had a similar problem with my wallstreet. Turned out the PRAM battery wasnt connected inside the machine, and as i dont have battery (constantly run off mains) the PMU was being corrupted every time I switched on/off, causing the symptoms you describe.
Appanently (read somewhere on net?) if the laptop battery dies and it hasn't been plugged in for a while the PRAM battery will go flat (causing the green light and fan noise) , but it can be recharged by leaving it plugged in with no battery for upto 48 hours. Once i'd reconnected the PRAM in mine it recharged over the course of a few days while the laptop was sleeping.
It is a bit of a 'long' job to get to the PRAM battery on a wallstreet, but follow the guide at
http://www.ifixit.com and it shouldn't take you more than 45 mins with a steady hand and patience
Hope this helps, if not i'd go with the logicboard suggestion, or find another Powerbook
*edit*
When I had this problem, it took aprox 2 mins for the machine to kick in after the fan noise.
also I had regular 'lockups' when inserting pcmcia cards under 10.2.8 so did clean install of 10.3.9 using xpostfacto, but thats another story...