Many problems on my 99' GTP

I bought a Grand Prix GTP about 2 years ago and have been having many problems with it lately that have been getting progressively worse and worse. First off it is hard to start cold and warm after it sits for about 15 minutes. Im pretty sure this is a leaky injector but I am not one hundered percent. There is also a loud whistle sound at low speed and the exhaust is loud and makes a clicking sound. I think that is a leak in the exhaust somewhere. It also has a very low idle speed and when it falls below 900 RPMs it makes a low grinding noise. I think that is caused by the bad injectors. It also seems to have poor low end torque but it varies all the time. Sometimes it has a lot of torque other times its very sluggish. I think this is caused by the injectors too. Basically what it comes down to is the car runs and sounds really poor. I have been doing a lot of research lately and have found a lot of different answers so I wanted to see what you guys thought. Any help is appreciated!

Welcome to AF.

Please post your mileage. Any SES/CEL light? Based on some of your symptoms it sounds like a possible CAT converter issue.

On the grinding/noise issue have you pinpointed where the noise is coming from? Check carefully around the supercharger.

Its has 130k miles and there is no service lights. The sound does sound like its coming from the supercharger but it had been replaced 50k miles ago, but i guess that doesn't mean its not going out.