thats the sound of the hammer striking the strings on a piano... most cheap keyboards don't emulate that sound and that's one of the reasons they sound so corny...
Now assuming that you are playing on a "professional-class" keyboard (like a Motif, Fantom, or maybe one of Roland's upper class keyboards, like the RD-700SX)....
Most keyboard sounds(pianos in this case) are made up of mulitsamples, which are samples of notes taken from a real piano over several key ranges, called key maps... In its simples terms, every key you play on your keyboard was taken from a recording of a key on a real piano..
On a good keyboard, there are hundreds of multisamples per patch-one sample for each note, and sampled at different velocities (the force at which the piano is struck)...
For Example, most mid-range keyboards have at least two velocity layers, which means that if you hit the keys with little force, you will hear a quieter, "darker" more mellow sound(taken from the sound of a piano played at a similar velocity). But if you play the keys strongly, you will hear a much louder, brighter sound. This is the keyboards attempt at emulating a real piano...
Now to the point... A really good keyboard will also contain a "hammer-strike" multisample, which was taken from the sound of piano hammers striking the strings... you will only hear this sample when playing the keyboard at a higher velocity (in other words, with more force)
If your keyboard allows you to tweak and customize instruments, you should go to your patch edit (or tone edit, or whatever it's called), and try to find a hammer multisample, then turn it up... you might have to read the your instruction manual to find all that stuff...
A more simpler way(but less realistic) way would be to layer your piano sound with a woodblock (or some other solid percussion instrument), and set it up so that the woodblock is only heard at a high velocity... but be sure to turn down the woodblock, so that it doesn't overpower the sound of the piano...
I probably confused you, but i hope i educated you a little bit!!!
