Soob98ImpOutSport

Noise (rattling, grinding, whining?) when I take my foot off the accelerator when the car is in gear, particularly at low speed in low gear. Depress the clutch or put it in neutral and the noise stops. 4th gear almost gone completely in that it slips out of gear, sometimes all the way, sometimes only partially. Mechanic says I have metal in the transmission fluid. So, I would like to know what part of the transmission is going out? Should I rebuild or try to find a used transmission? Which transmissions fit this model? Love my soob. We've been together since 2001. Please help me educate myself before I invest in this endeavor. Thanks in advance for your advice!