Anyone who made it to 10th-grade English class probably remembers studying onomatopoeia words. Those are words like murmur and buzz that sound like the things they describe. You might also remember sniglets. Those are words that don't exist but should, like pajangle (the state of waking up with one's pajamas turned 180 degrees). Well transnomatopoeia is a sniglet word. It is best defined as imaginary sounds that don't really exist, but if they did, this is how they would sound.
A classic example of transnomatopoeia occurs in Loony Toon cartoons. Think of Porky Pig tip-toeing across a scene. Of course, tip-toeing makes no sound. But we hear "plink-plomp-plunk" (probably played on a harp) on the soundtrack. Transnomatopoeia can be thought of as a way of snigletting sound effects. Inventing transnomatopoeia words is a bonmotoring activity, but anyone can play. Many do.