At those RPM the major change in the engine's operation is the turbocharger reaching full RPM, which also means full flow through the exhaust. You might find small black sooty deposits around part of the exhaust manifold, particularly the part that meets the turbocharger (although don't ignore the EGR pipe). Gaskets can become thinner and act like a reed in a clarinet say - I had that exact problem, and mine occurred mostly around 2,000rpm.
Otherwise it would be prudent to check that the pulley for the alternator is firmly attached to the alternator, and the radiator fan itself doesn't have any cracked blades that might be pulling forward at the higher RPM and scraping the back of the radiator.