USB Sound Card Static

I currently have an iMac (21.5 2014) and since there is no line in port, I had to buy a separate USB sound card to plug in my headset including its mic, When both audio in and out cables are plugged in the sound card, there is no white noise when recording with the mic however when I plug my mic only and plug my audio out cable to another source to listen to the other monitor, there is white noise in the background when recording with the mic. Is there a way to eliminate the white noise or will I have to waste more money on another sound card etc. after spending over a thousand on a computer that doesn't have a line in port? (This is also making me want to sell my iMac and buy a PC now).