If it was a stone or nail in a tyre, the ticking would increase with speed, not stay constant as stated. Also, you should be able to hear it even at very low speeds, such as cruising through a carpark. I have had rocks caught in my tread before and it was most noticable when driving slowly.

I would say its something loose, maybe the under tray? Leak in the exhaust manifold?
I have had a leak in my exhaust manifold before and it would tick pretty loud when driving next to a wall.

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"Wrong fitment".