DO NOT DISSASEMBLE THE FRONT STRUTS WITHOUT A SPRING COMPRESSOR. THAT SPRING WILL GO THRU YOUR ROOF AND YOU FIRST IF YOU'RE IN THE WAY, NOT KIDDING
if everything looks good replace the front upper strut bearings/mount (same item). They contain the isolator bushings which take up small sharp bumps and ruts before the shocks have time to react and will weaken early due to them being so soft. I had one removed and put in a press and the rubber was peeling away, so they were converted to urethane, haven tried them yet. One thing you may want to try if youre handy with a hydraulic jack is to place the front of the vehicle on jack stands (don't place the stands under the suspension) and remove the front tires, place the hydraulic jack under the front control arm under the ball joint if possible, jack the arm up a few inches and release the hydraulic jack quickly, if you hear the same thunk noise its the from the upper strut mount bottoming out.
When initially hitting a speed bump the top of the strut rod will push the upper section of the strut mount into the shock tower due to the shock being stiffer than the bushing, on rebound the bushing will bottom out also causing the double thunking noise, not saying this is it, but might be.
The other way to check them is to place a video recording device with a light under the hood with the rubber plug on the top of the front shock removed and drive over bumps, the noise should correlate with the top of the strut rod moving.