Hissing noise when I put a tape in the dictaphone - with or without hitting play. No issues with functionality.

SOURCE: Dictaphone 3750 transcriber

If it does this with all tapes, there is a small "no-cassette" switch at the back of the cassette compartment. The switch may be broken, dirty or out of adjustment. When you open the cassette door you should be able to see the switch tab. Push it in just a bit to see if the No Tape indicator goes out.

SOURCE: Dictaphone indicates end of tape

The sensor that detects the tape movement is bad and will need to be replaced.

SOURCE: Dictaphone Expresswriter 2740 Tape Won't Move

The problem is with the capstan motor. More than likely it has developed a 'flat spot' and won't spin up. The clicking noise you hear is the solenoids trying to pull the heads into place, but because the motor isn't turning, it can't pull the heads into place.

SOURCE: Dictaphone ExpressWriter system - tape will not rewind, play, ff

Is this a standard-size cassette unit? Is the model number a 2740 or 2750? When you first plug the power in, does the machine make two clicking noises? Does the display show 'CAS' or 'No Cassette'?

SOURCE: Dictaphone 3740 started to hum but would not play,

If you press Play, Rew or FF and can hear something in the machine (even though the tape doesn't turn), then it's more than likely a broken capstan belt.