look inside the spring. you will see a "cone". get under the car and have someone bounce the car up and down and see if the noise is coming from there.

 

Had the same squeaky noise like an old mattress from the rear. It came and went depending on temperature or humidity of the day. It was easy to reproduce just pushing on the rear. So we got the car lifted and tested. Sure enough, the lower suspension arm (a short piece) was the culprit. While it was structurally intact and not posing any danger, it was noisy. Spraying and lubing its bushels and the joint area cured the embarrassing and annoying noise. The part itself is not expensive to replace, and whenever there's a chance it should be done. Probable cause was the winter salty and slushy snow. It must've got into the assembly causing terrible friction.

 

Don't be surprised if the bushing starts squeaking again in a month or two. You can re-lubricate it every few months when the squeaking resumes, but if you plan to keep the car for a while, the long-term fix replacing the arms (both sides) and their worn bushings. Like you said, not too hard on your or your wallet.

 

Hello fellow NE Ohioan. I used atp at-205 on our STR bushings and the stuff works amazing. It was about $10. I guess it last for about a year before needing to be reapplied. Wish I would have used it before replacing a lower control arm bushings. It really made the STR drive better after using it on all the bushings.