Could just be reverberance at them revs causing the heat shield to rattle. The ST170 cat has a couple of heat shields surrounding them, these get really hot a corrode so tend to come loose and rattle at certain revs. Get underneath and have a look, the Cat brackets tend to shear also and need welding. I would not be ovely concerned at this moment in time. Most likely the heat shield.
Some thing else i would check on the ST170 is the IMRC (inlet manifold runner control) module. Just as a matter of course. There is a silver box bolted to the fire wall at the back of the engine. When you turn on the ignition it pulls a cable which shifts the runner in the inlet manifold, then when you hit 5000+rpm it releases the cable and the runner goes into its second position. The cable is attached to the passenger side of the inlet manifold under the throttle body, check that the cable is attached and working as the level on the maniold tends to snap off. Just watch the cable as someone turns on the ignition, it should move the runner as soon as its turned. Any clicking sound coming from the bulkhead area will be the IMRC unit its self is knackered. these are very common problems. Along with jerky driving at high speeds and loss in power is usually a faulty fuel pump. The only other real BIG issue with the ST170 is the dual mass fly wheel, they die very quickly around 60'000, this will result in a new fly wheel, clutch, clutch cover plate and slave cylinder which will cost you £1000+ This will hapen at some point, my ST170 just his 59'000 and i just paid out £1000 to have the whole lot replaced!!
Good luck.