240 wagon, 350,000 km on the car.

The back end 'ticks' or clicks (best way I can descibe it) constantly when driving at any speed, even on smooth roads. It appears to be from near the top of the shocks, and comes from both sides. Bouncing the car will cause it with only about 1/4" movement, and it mostly happens on the rebound (every rebound). Shocks are two years old ~30,000 km. Boge Turbo Gas. The mounts are tight. Also replaced all the suspension bushes except the big ones under the axle last year and they look ok. The noise doesn't sound like something's loose or rattling, just moving or rubbing. Any ideas?