Chevrolet: Coolant Light and rear noise, 2000 chevy cavalier, cv joints

Question
I have a 2000 Chevy Cavalier 4 Cylinder.  I have had a problem with a knocking sound in the rear of the car for several months now.  I have had all 4 tires replaced.  Both front and back brakes are replaced as well.  But there is still a knocking noise when I lightly step on the brakes or when the car is slowing down even without the breaks.  Three different mechanics have said that there is nothing wrong with my shocks, axles or CV joints.  Do you have any ideas?
Also, this morning my coolant light flashed on then about 2 seconds later went off.  I contacted the local Chevy place, and they said they would need to run a diagnostic and flush the system to find out what the problem is.  I checked the fluid levels and they seemed fine.  I have noticed no anti-freeze smell and there have been no signs of a leak under the car where I park either at work or at home.  This morning was much cooler than usual however.  Could that be the cause of the blip?  I don't have the money for all of the work they are talking about doing on the coolant system.
Thanks for your help!!

Answer
Valerie;
      Hi - if you had it looked at  and they found nothing , did you check in the trunk - how about the spair tire ,or the ex.system , muffler ?
as far as the coolent - if the level is good and you are not running hot , it should be fine. The light  went out  so it just could have read  wrong for a sec. as long as the light is not on all the time.you should be fine but I would look in the trunk and check the ex. system...>>