The Tdi Pro has an unsteady threshold, notorious for it. I run with the threshold off when on the beach, with a gromit under the knob so it only moves when I want it to. When prospecting I run with a threshold, due to mineralization and the ground noises which go with it. If you are just at the point of no threshold then ground noises will intrude, as will targets. My experience in hot ground run with a threshold, silent when on the beach. You can prospect without the threshold, just turn it down a bit further and to compensate, up the gain a bit. Combine this with a slightly positive ground balance and run in LOW conductivity. All depends on the ground really. When starting out it is sometimes a good idea to use a test nugget and 'play' with the settings to see how the machine behaves in different settings. I gave myself a few days on the goldfields where the only objective was to 'play' and learn the machine, finding gold was secondary. As experience builds you will find more and have more fun doing it..
The Pro is all about warbles and that has some benefits, you can also push the threshold higher past the warbles and use the volume control to make it tolerable, headphones will alert you to any small changes signalling a target, goes back to taking some time to experiment and see what works best in your location and suits your style of detecting.
Again all the best, Karelian.