Original Koni shocks have a design fault which can lead to a clonk clonk noise.
When this happened to me I thought it was the coolant tubes, as it sounds exactly like the noise you expect from the tubes or radiator being loose.
So if it isn't the rad or the rack and you have original Koni shocks. Remove them and check that they operate smoothly and evenly. If they don't, then get them rebuilt (it should cost much less than new replacements).
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