Dribbling eco drake

I recently installed an eco drake and it makes a dribbling sound in the trap after it's finished flushing. It lasts for 30 seconds to almost a minute. I called Toto and they seemed clueless about it except that it seems like it is overfilling the bowl and equilibrium causes water to spill into the trap. But they couldn't tell me why it does that and in the eco drake you can't adjust the level in the tank. The Toto Clayton I also installed recently is quiet.
There are two things I can think of that may be possible causes. One is that the floor is about 1/8 inch lower at the back of the toilet and the other is that my water pressure is high, especially on the ground floor where the eco drake is. I know the noise thing is a minor problem but it just seems weird. Any thoughts?

All toilets overfill a little. SOme do it a lot. The less water they use, the less overflow. It probably isn't a problem at all. Make sure that the hose routed to the overflow tube is not too low - it should be above the top of the overflow, not down in the pipe.

I think you are right in your suspicions about the unlevel floor. If the bowl is higher in the front than in back, water will trickle into the trap after the refill. Check the bowl for level front-to-back. Plastic toilet shims are available. A new wax ring would be best since you don't want to compress the wax and then raise the bowl up again with a shim. Fit it dry and get your shims in place, then install it with the wax ring.