nocking from back of car could be a shock bush

[tintin]

for a while i have heard a nocking from the back drivers side of the car thinking it was the exhaust but i can not phisicly make that same noise when o more the exhaust ????

my dad said when he hears the noise he can feel it through the strut brace could it be the shock bush? (ive got cusco z2's or was it z1's??)