2000 Buick Century, 95k miles, purchased six months ago.

A month ago I installed new KYB bare rear struts and re-used the insulator, coil spring, top mount (includes metal tubular guide/bearing), and washer. New nylon top nuts came with the new struts. I used a rental spring compressor from a parts store and did not use and impact gun for compressing the springs or tightening the top nut. After installation they were making a popping sound so I tightened he top nut down (car on ground and me in the truck) as much as I could with a go-through socket and allen wrench to prevent the shaft from spinning. I also tightened down the three mounting nuts to the body as much as possible but the popping noise still continued. This was my first time installing struts and though I marked the old ones for indexing prior to removal I figured I must have messed up the indexing or a worn mount/guide is causing the popping noise.

I removed the new struts that I assembled and brought them to a mechanic that installed the new front struts I purchased soon after the rears. He has a wall mount spring compressor. He decompressed the springs, indexed (clocked) the spring correctly to the insulator and top mount, installed the top washer correctly and then used an air impact to tighten the top nut. He said that the indexing was off on both rear struts. He told me when installing to preload the struts (which I did prior) and to try and tighten the top nut as much as possible during preload and again when car is level on the ground.
Unfortunately, the popping noise is still present so I intend to purchase new top mounts for them and try it again.

I am positive the old struts I removed were factory GM struts (car has 95k miles). I made sure to take pics of the parts during disassembly and there is only one washer (top) per strut. I also confirmed this online via GM dealer parts diagrams that there is only one washer (top) per mount. However when I search for rear struts mounts online some of the cheaper aftermarket brands such as Anchor, Duralast, Westar show two washers per mount in there images. AC Delco, KYB, Moog only show one washer per mount. I did mention this when the mechanic was assembling the strut and he said there is typically only washer for GM struts.
I'm not sure if the difference between using on or two washers will solve this popping noise so that's why I mention it here?

My inclination is to go with either AC Delco or (most likely) KYB for the new strut mounts. I hope this solves the popping issue but the new KYB strut's shock shaft seems to be a very tight fit in the old top mount guide/bearing. If there is free play I can only guess it's very minute. Before I buy the mount does anyone have a suggestion to try tightening down the top nut one more time to hopefully solve the issue? Some people in various forums online said they had a helper use vice-grips or channel lock combined with a thin piece of thin rubber (to avoid nicks and scratches) wrapped around the shaft to hold the shaft while they tightened the top nut with a socket wrench. Most said that they were able to turn the top nut another quarter to a half turn more and that stopped the popping noise. However, KYB (and probably all other brands) installation guide implicitly states not to use vice-grips or channel locks on the shaft while tightening the top nut.