(1) Rear shocks or rear strut cartridges are worn or faulty.
Before explaining this, we'll need some background info on rear suspensions. A rear suspension has two main components - shocks and springs. On most vehicles, these two components are separate from each other, on others they're part of a unit called a "strut". The "shock" portion of a strut is often called a "strut cartridge".
In either case, the idea is the same - the spring absorbs irregularities of the road and the shock/strut cartridge keeps the spring from "bouncing" too much (i.e. it restricts the spring's contraction/expansion).
Now, back to the problem at hand. When a vehicle passes quickly over a dip or bumps, the entire weight of the rear end sort of "comes down" on the rear suspension components. When the shocks/strut cartridges are worn or faulty, they can't handle the weight. Consequently, they're compressed to their maximum or, more loosely, they "bottom out". This results in the loud "thump" you're hearing or sometimes a rattling noise.
What to do?
First, you'll need to determine if this vehicle has shocks or struts in the rear. If shocks, having them replaced should correct the problem. If not, this condition could lie with the springs but that's unlikely since springs generally last the life of the vehicle. If this vehicle has struts, you can either have the entire strut replaced or, in many cases, you'll be able to simply have the strut cartridges replaced. Call a local dealer's service department and ask if the rear struts of the make/model in question have replaceable cartridges.
When to Replace Shocks and Struts
When to replace shocks and struts? if your car encounters front end dives when applying the brakes then it means front shocks or front strut is faulty.Learn more: When to Replace Shocks and Struts
To confirm worn or faulty shocks/strut cartridges do the following - step down on the rear bumper several times to get it bouncing and let off. If it continues to bounce more than once, the shocks/strut cartridges are worn/faulty.
Please take care of this problem very soon. Why? Because, with each "thump", certain rear-end components are becoming more damaged most notably the "bushings
