I want to throw my experience on here for those who may experience mysterious rattles/knocks/thumps and want to restore peace and quiet in their Prius, and as a thank you to all the posters and advice found on this site. I've had a knocking or rattling sound coming out of the front driver's side strut area on my 2012 Prius for a number of years, and was told by my local "Prius specialist" shop that I simply needed new struts and quoted me $1700 in late 2018. This sound was difficult to locate but occurred both with steering and when going straight, worsened by any pavement imperfections, and occasionally becoming more of a rattle if enough vertical movement was introduced into the suspension (e.g. going over a series of bumps like rail crossings). After monitoring it for a while and not noticing any worsening sounds or steering/ride deterioration, I decided to suck it up and live with the annoyance until I could get around to replacing the struts myself as my wallet did/does not runneth over, so to speak. The pandemic afforded me that time, and I promptly have spent most of it doing all manner of other things, until this weekend when I finally picked up a replacement set of assembled struts and swapped them in, eager to hear sweet cruising silence upon finishing the job. However my test drive on the new strut assemblies gave me the same knock and rattle. Damn. It was only then that I dove deeper into the forums and youtube to suss out other culprits, and I came upon the usual suspects including the stabilizer bar, any of the bushings in the system, the intermediate steering shaft, or the steering rack. However I also found mention of the loose bolt on the steering column that seems common in Gen 2 and Gen 3 Prii. This is the easily accessible 12 mm bolt on the steering column under the steering wheel, next to the universal joint. I thought why the hell not give it a go, and 90 seconds later and with a quarter turn of a ratchet, what I thought was a suspension rattle had vanished. The moral of this (long, sorry) story is: make sure to do adequate triage and pick the little battles before diving into big fixes (and don't blindly trust your local shop, good intentions though they may have). Or just be better than me at diagnosing what's loose by ear. New struts aren't gonna hurt anything at 110k miles, and the experience was valuable, but I kick myself a bit knowing that many months of annoyance, the better part of a Sunday spent wrenching, and hundreds of dollars all could have been saved with a quick dive under the steering wheel and a bit more searching.
I have had 2 mysterious episodes of rattling as you've described. Both on Gen2 cars, one a 2005 and another 2008. I still have not gotten to the bottom of it yet. I've checked the bolts you've referred to, but they were nice and tight. On a Gen3 2010 I have, I did have a intermediate shaft that wore out, that was replaced by Toyota and any looseness in the steering column went away.
I'm not as familiar with the Gen2, although I do see that others have had the same issues with the bolt I described. Wish I could be of more help. I'm guessing you've had it up on jacks/a lift and played around with the steering and suspension piece by piece? That was what eventually allowed me to better hone in on the source.
Congrats on fixing the rattle! I will give it a try as well. Out of curiosity, did your rattling sound like this?
Sorry for the months later reply, I don't check priuschat too often. It's hard to compare the timbre on a recording, but overall it did sound similar, with those moments of multiple knocks/rattles followed by silence. I'd guess likely the same problem. As an overall update, 5 months and a few hundred miles after tightening the bolt and thinking the issue was put to rest, the rattle returned with no obvious trigger. Diving back under the steering wheel I found that the bolt remained tight, so I actually loosened it a bit and gave the column a wiggle before tightening it back up. That fix lasted only about a week and <100 miles before the noise again returned. At this point I don't love the idea of frequently messing with the steering column, so I'll chalk it up to a manufacturing defect and monitor it for progression, but my guess is it will remain an annoyance until the car and I part ways. Knowing that the noise-making part is tight and doesn't appear to be functionally impaired is somewhat reassuring, but I certainly think that Toyota's reputation for reliability takes another stain given what seems to be a not-uncommon problem with an extremely important system from a safety standpoint.
