A hammer?

 

I'm guessing it was nothing more than a synthesized noise. Something generated by playing with the attributes of an existing sound, like something on a keyboard.

This is one of those great mysteries.

I was going to say "no it isn't, it's just an electronic tone."  But, well, it turns out things are not that simple after all.

Composer Mike Post wrote the theme song using electric piano, guitar, and clarinet. In addition, scene changes are accompanied by a tone generated by Post. He refers to the tone as "The Clang," while Entertainment Weekly critic Ken Tucker has referred to the sound as the "ominous chung CHUNG", actor Dann Florek (in a promo) as the "doink doink", and Richard Belzer as "the Dick Wolf Cash Register Sound." The tone moves the viewer from scene to scene, jumping forward in time with all the importance and immediacy of a judge's gavel – which is exactly what Post was aiming for when he created it. "The Clang" is an amalgamation of nearly a dozen sounds, including an actual gavel, a jail door slamming, and five hundred Japanese monks walking across a hardwood floor.

Holy cow! I wasn't too far off in my estimation. Though who could have guessed 500 monks and jail doors were involved? 

And here I thought it was a gavel hitting a metal pipe.