I owned one for eighteen months or so. No loud knocking noise from the roof on mine, that sounds like a problem. You get rattles from the roof cradle stowed in the boot, but easily solved with little rubber door wedges shoved in to stop the rattle.
It's a very comfortable grand tourer, too heavy to give blistering performance. However, a spirited drive is possible, particularly when using the gear shift paddles. All of my mileage was done in SW Wales on hilly, bendy A roads.....I had some fun.
I only changed it for my RX because of back trouble for which I needed an easier accessible vehicle.
Buy an IS250C, you'll have some fun, especially when the weather is good.....still miss mine.