Hey guys this is my first post and would just like to see if anyone has any advice or ideas what is going on with my bike before I take it to a mechanic. I just got a 2000 CBR F4 two weeks ago off of craigslist. I have owned bikes in the past but this is my first 600. I bought it from the guy when it was already warmed up (I know bad idea in hindsight) so I didn't notice any issues when it idled. Once I started to ride the bike home however, no matter what gear I was in, the engine would make a strange noise whenever I held the bike in the 4-5.5 rpm range. The best way to describe the sound is a deep "wawawawawa" as I go through or hold in that range of RPMS but below 4 or above 6k RPMS the bike sounds totally normal. I have not been able to find a single post or video on YouTube with someone having this noise coming from their bike. And when the bike is making the noise there is no loss of power felt and the RPMS don't waver at all, from riding and looking at the gauges the only way to know the bike is doing something strange is by hearing the noise.

Also, when I got up the next morning I tried to start the bike up and it immediately died. I had to open the choke all the way and keep throttling the bike because it was holding about 600 RPMS. Once the bike has been rode for about 5-10 minutes however the bike idles at 1500 perfectly. I think this could just be that the bike is running lean or some other issue with the spark plugs/fuel jets. But it still doesn't explain why the bike is making the strange noise.

Thanks for any suggestions!