Well, the title says it all. It's not anything I'm carrying in the trunk and with a full tank, it still makes the sounds on each bump in the road. Any ideas? Low muffled thumps.

 

Sounds like it could be suspension related. At your mileage your bushings and dampers should be still good, however depending on road conditions, wear can be accelerated. Have you had any of these parts changed or serviced and if so, when?

 

You did check the spare tire well?

 

The title says almost nothing useful for diagnosing the source of the sound. Does it sound like it is coming from inside or outside the vehicle? Is it loud or soft? Has anyone sat in the back to try to locate the sound more closely? Does it sound metallic? Is it a single “thump” or more than one at each bump? At what speeds does the OP hear it? How big of a bump is needed to make the sound? How many miles on his ride? Any work done recently, like brakes or new tires? It could be a ball joint or other suspension component, but throw us a bone.

 

I'm new to this so it was just a start for me. It sounds like it's in the area of the lower right rear quarter panel. As to an extra rider to check it out (65 and 66), at our age, that'll be a new experience, but we'll try it. I'll also check the spare tire to see if it's battened down well or not. Thank you all for your suggestions and I will start looking tomorrow. Oh yes, it's a recent purchase so it has coverage and we'll either solve it Saturday or the dealership will next Thursday. We'll let you know what we discover as soon as we know more.
Tom

 

Well, well, very well! Unbeknownst to us, the new/used 2018 Mazda CX-5 evidently had new rear struts installed and the passenger side top nut just hadn't been sufficiently torqued. A little Loctite and retorque solved the thump. Nice going Toyota of Iowa City, Iowa. And no charge is why we go back there! They have been good to us for many years.
Now a new question. Has anyone found an easy way to get a wire from the engine compartment, battery side (Driver side) to the driver's compartment for a new switch?