So yeah I had an issue in July regarding my forester and found out that I was lied to by the sales man and need to put 93 octane in my ride (only nowhere in town sells 91 octane or even 91, only 90) so a bottle of octane booster(that doesn't contain MTBE which voids warrenty). Well the other day I had an issue where while accelerating (DOWN A HILL) I went from 10% to 20% throttle and had a loss of power and a completely rhythmic "tap tap tap tap" (about 3 or 4 taps per second) from the rear of the car, like something was in the spare tire well (aside from trunk gun, first aid kit and spare tire), I let off the throttle and the issue went away and I said to myself" nahh I imagined that maybe if I ignore it , I wont experience it again..." well it happened again this week different speed 30 going to 40, mild acceleration, maybe 25% to 30% throttle, level surface, and same "tap tap tap tap tap tap" about 3 or 4 taps per second and distinctly from rear of car. I don't know what is wrong. Any suggestions?

So had it happen again on the highway when I was passing someone, (going 65 wanted 70) so light press on pedal and went into passing lane and "tap tap tap tap" awesome to go from 250 HP to 100ish. almost got rear ended... tap tap tap sounded like it was from under hood so now im not sure if It was me thinking it was rear last couple times or the noise is actually moving.