The top of my left rear shock has some play in it, it rattles when I go over bumps. When I rock the rear end side to side, it will rattle. If I grab the shock I can feel the play. My mechanic seems to think it is fine if I don't mind the rattle. What do you think?

Check the shocks!
Bruno

I had an issue with the rear sway bar links that caused a knock and rattle, the sway bar keeps tension on so it is hard to feel for play.

try to duplicate by rocking the front end of the vehicle up and down and you may be able to spot whats loose tec.

usually its the sway bar links that rattle horribly,it sounds like the suspension is falling out...but its not, the sway bar links are usually pretty cheap and not bad too replace,if you can get under the car grab them and give em a good pull ans u will usually feel a little play in the ball and sockets on the ends of them,hope this helps,or have your hand on one and have a friend rock the car side to side and u might feel it then

If it's making a clunk noise when hitting bumps, check the mounting plates and bushings that hold the rear end to the frame. GM made these plates out of aluminum and the bolts that go through the plate and the bushings rot out and the rear end slams against the frame.

strut top mounts bad or rear sway bar bushings. always one or the other.

the shocks will make a noice ,check each one from the top.they will have a rubber around them,if it is gone it will rattle.shocks are very expenseve.

shocks

It could possibly be a shock or strut bracket.

If Jay's tips don't pan out, consider having your shocks checked since these can produce a loud rattle if the seals are blown.
A preliminary check can be made using the 'bounce' test. Pressing down and letting up on a fender several times in succession until the car is bobbing good. When you let go on a downstroke, the car should come up once, settle and move no more. If it kind of fades, with several shorter bounces, the shock is bad.