I own a 2008 Euro Civic Type-R which has almost the same engine as the US Civic Si (K20Z3 vs K20Z4).
Since a bit of time I have a really annoying high pitched noise coming from the engine. It's constantly present, completely tied to revs, and persists when clutch is disengaged (so it's not coming from the gearbox). It sounds a bit like an electric motor from a radio-controlled toy car.
About six months ago (maybe a year) the sound could not be heard inside the cabin when cruising unless windows were open, but started being heard when opening the windows, especially in an underground parking lot. I brought it to the dealer who said it was normal engine noise, and that "fixing" it would require a new engine (quotes meaning that it was, to him, not requiring being fixed).
The sound is stronger now as I can hear it in the cabin when cruising with windows closed, to the point of being distractive, if not painful on long journeys, so I don't really buy it that this is normal.
THe car is completely stock. I'm running on 0w40 oil, am regularly pushing the car but have always respected warm up time, and since I bought the car new, did (to the best of my knowledge) proper break-in. FWIW the K20 engine has a chain transmission and an accessory belt.
Here is a video recording where I rev at a standstill, with clutch disengaged (watch in 720p HD for clearer sound).