Transmission buzzing. help!

I have a 1991 Mitsubishi eclipse with the 2.0L and automatic transmission. i recently had my tranny stuck in 3rd gear but i replaced the tcu and that fixed the problem. so now it shifts but kind of hard. sometimes when it is cold, it will not engage in drive and i have to put it into a lower gear. but after i put the new computer i turned the car to on with the engine off. and i put it in gear. there is this weird buzzing but when it is in neutral or park, it stops. i think it's possible its a bad transmission solenoid but i am not totally sure. anybody got any ideas? The buzzing was not there before i replaced the computer. but the shifting problem had always been there.