Whenever I lock the screen, after a certain amount of time, the cpu and the cpu fan use go up near the limit, and it returns back to normal the moment I unlock the screen again (as shown by psensor).

How can I find out what process is responsible for this?

I've tried with "top -S", and the result is this...

  PID USER      PR  NI  VIRT  RES  SHR S %CPU %MEM    TIME+  COMMAND    
 2114 me       20   0  326m 104m  40m R   16  2.6  66:50.03 compiz     
 1234 root     20   0  396m 152m  98m R    6  3.8  20:23.88 Xorg       
 2204 me       20   0  160m  38m  30m S    4  1.0   0:33.35 yakuake    
 2446 me       20   0  206m  18m  12m S    4  0.5   6:32.18 psensor    
 2280 me       20   0  220m  18m  10m S    2  0.5   5:01.60 unity-panel
 9138 me       20   0  154m  27m  15m S    2  0.7   0:03.63 plugin-cont
 2282 me       20   0 65800 5272 3316 S    1  0.1   4:36.90 hud-service
 2143 me       20   0  140m  11m 8352 S    1  0.3   2:50.16 indicator-m
 9095 me       20   0  720m 253m  36m S    1  6.4   0:26.34 firefox    
 2076 me       20   0  7168 3484  828 S    1  0.1   1:46.53 dbus-daemon
 2307 me       20   0 55000 5132 3632 S    1  0.1   2:01.55 indicator-a
 2557 me       20   0 86328 6028 4576 S    0  0.1   1:44.71 conky      
 6290 me       20   0  2836 1296  964 R    0  0.0   0:29.64 top        
 6291 me       20   0  2836 1188  884 S    0  0.0   0:29.49 top        
   1 root      20   0  3644 1984 1284 S    0  0.0  60:57.76 init  

specs:
ubuntu 12.04 fresh install
intel core i5
4gB ram