TICKING BOMB
This circuit produces a sound similar to a loud clicking clock. The frequency of the tick is adjusted by the 220k pot.
The circuit starts by charging the 2u2 and when 0.65v is on the base of the NPN transistor, it starts to turn on. This turns on the BC 557 and the voltage on the collector rises. This pushes the small charge on the 2u2 into the base of the BC547 to turn it on more.
This continues when the negative end of the 2u2 is above 0.65v and now the electro starts to charge in the opposite direction until both transistors are fully turned on. The BC 547 receives less current into the base and it starts to turn off. Both transistors turn off very quickly and the cycle starts again.
source : http://www.talkingelectronics.com.au/projects/200TrCcts/200TrCcts.html