Evan points you to a good place to start learning. Read that. There are several companies that make the stuff. There are several bake-offs between the brands on the web. The most expensive and well known is not the best.

To start, tighten up the loose things. That is free and easy. Put a heavy rubber mat over that 6'x8' bed that amplifies road noise like a giant drum!

You can soundproof the floor on the top, below the carpet and below from underneath. That will isolate much of the tranny and road noise.

It is not that hard. You can do it in steps over a long time. I have a bad back and I did with the help of a couple of conversations with the owner of a manufacturing company. There are several sound proofing forums.

There are so many things you can do in small steps. For instance, there is material that passes heat, but not certain frequencies of sound. It is not affected by engine heat. Paint it or trowel it on the underside of your hood. The heat will still go out through the hood but not the noise. You'll knock out about a third of the engine noise. You can use a cheap special paint gun to spray it if desired.

I probably have more than my own weight in materials and $400 dollars invested. Each step does a little until you realize it is quiet. Just a little bit of time and money at each stage. There are several types of materials put down in layers, each layer acting on a different set of frequencies.

I consider myself only 95% done. I still need to decide if I want to put more material on the engine side of the firewall. I might never do it.

I'll try to dig up those web bookmarks for you later this week. I'm having a root canal done first. Please be patient.

 

I like how a thread on tweeters turned into a soundproofing thread. I'm not sure about the OP, but soundproofing is not for me. I am eventually going to put tweeters in my truck, but not just to overcome the sound of the road/engine. I simply want tweeters in my truck. Some people may like dead quiet, but if I love hearing my truck. Anyways if its above 50 degrees outside I'll usually have my windows down anyways.

Anywho, my cousin has an 89 ranger, and he mounted his tweeters in the door panel next to the door handle (on the rangers the door handle was more up and closer to the dash). Just a though.