So I replaced the front struts with KYB complete struts in my wife's highlander. I know, this is a 4runner forum, but it applies. About a month after, when she made a turn, a weird fluttering noise emitted from the front passenger side dash. For the life of me, I couldn't find it. I thought..steering rack? but the rack is manual, it has electric pwr steering.
So, out of frustration, I bring it to toyota. This is after checking my install. They say tne problem is the strut and I need to replace them with toyoto struts for some crazy amount of money. So, i tell them I just put in two brand new struts. They check it again, they say its the passenger strut making a harmonic. So, $139 buck later, i take it home. I take my car stethescope and put it on the top of the strut while she turns the wheel. Sure enough, its the darn strut bearing/bushing...whatever they got in the top of the thing. the other side was silent, by comparison.
I order a new strut from Rock auto, cause they won't send a new one without the old one back first, unless you buy one and then send the old one back.
I put the new one in...silence. Not a sound, no noise, perfect.
So, the new KYB was defective. Rock refunded me the part, but I ended up out he $139 and half the shipping. Just hope no one else has to go through this.

Thanks for the time, effort and diagnosing the problem. Also posting so others would know.
I've never heard of it so it's good to know.

strange issue for sure. i installed the KYBs on my 4runner with no noises or issues at all. but as with any part, there can always be a defect or error in the installation.

however, a coil over shock, not a strut, would not make any noise when steering, the shock doesnt twist or move at all. a true strut may be different & being a highlander with front struts that could happen, but it doesnt apply to 4runners that dont use struts.

could also have been an install error or assembly issue, i know you said you "checked the install" did you check all the mounting bolts & the big center top hat bolt that holds the spring for proper torque specs? symptom almost sounds like something wasnt fully tightened. but doesnt matter now since you fixed it & rockauto/kybs lifetime warranty replaced it minus the shipping charge & diagnostic fee from the dealership... in the future i suggest staying as far away from a dealership as you can unless still under warranty, $139/hour that took them 15 minutes to diagnose then try to sell you their struts for a crazy high price shows that "most" all dealerships are a complete ripoff!