The squealing on cassettes is caused when the lubricant in the tape evaporates or rubs off from excessive use. When there is little to no lubricant on the tape, it causes a high pitch wailing noise when played. (Sort of like a kettle) It's very annoying, so I will teach you how to fix this problem easily.
Step 1: What you will need
&Bull;A squealing cassette
&Bull;A portable cassette player (particularly one that opens while you're playing the tape)
&Bull;A clean cloth
&Bull;A cup (optional)
Step 2: Step One
Rewind the tape all the way back so that it's just about to play side one. (or A) When it's done, push stop and open the player.
Step 3: Step two
Find a spot in the player where you can access the tape. On my Walkman, there is a single hole in front of the cassette where the actual tape is visible. Check to see if that opening's big enough for a q-tip. If not, see if you can find something smaller.
Step 4: Step Three
Tip back the vegetable oil bottle just enough to where you can get it to soak one end of the q-tip. (Or you can put it in a cup.) Let the tip soak for a few seconds. You want it to look big and fat. Put the q-tip in that little hole. Press fast forward.
Step 5: Step four
Hold the q-tip in that spot while it fastforwards. If you can feel slight vibrations from the tape on your q-tip, that means it's working. You may want to hold the top of the tape do that the tape is closer to the q-tip.
Step 6: Step Five
When the tape is done, hit stop. (if it doesn't stop for you.) Flip the tape over. Put it back in the player. Get a new q-tip and soak it in oil. Put the oil in the hole and hit fastforward again.
Step 7: Step six
When the tape is done, take it out of the player. Wipe everything off with a clean towel. Play the tape to see if I works. If it does, than hooray you're done! :D