1992 Chevy Silverado Whining/Whistling Noises

Every morning I turn on the truck, it starts making like a whining noise. And it would do it throughout the day also. And it's been making this noise for about a month and a half now. It's pretty loud when it's at idle but once I give it gas, it gets a little quieter. At first, I thought it was the serpentine belt or a pulley but it's not. I asked my father and he told me that maybe it's the alternator. But it's not draining the battery so I haven't taken it to Autozone to get it checked yet because I know how some businesses just say it is a part and it really isn't. So I'm still asking around before I actually start spending money on it. My father and some friends and I were looking at it to try to figure out where it's coming from and my father got under the truck, because we were hearing the noise from the back of the motor by the firewall. It sounded like it was coming from underneath the firewall and I asked him if it could be the flywheel. He touched the cover of it and the noise got a little quieter. So now I'm thinking something is loose down there. It does not leak anything, the temperature does not change, it runs perfectly fine. My father owned it since 1994, until the Christmas of last year when he gave it to me as a gift, and has kept it extremely clean. He has not really changed major parts, that I know of. I want to say that 95% of the truck is still original, so I'm thinking another part is giving out after 130,000 miles. In the middle of last week, the noise has been a little quieter and it's scaring me because I don't want something bad to happen. I hope you have heard of a case like this before. To minimize my spending. What do you think this noise could be? Should I stop driving it? Because right now it is my daily driver. Thanks for your help.