The first one tells us that you stopped playing when the bell rang. The second one doesn't tell us this. Maybe you stopped playing or maybe you continued playing.

Hi, The difference is none existent. Ordinarily, the past perfect simple / past simple combination is used to clearly establish the time sequence of the two activities. The activity in the past perfect happened before the activity in the past simple. The past continuous / past simple combination is used to describe a long duration action (PC) being interrupted by a short time action (PS). In your case, you've got the past perfect continuous and past simple in your first example. So, your playing wasn't clearly finished when the bell rang. I would argue that there is no difference. Cheers

We had been playing when the bell rang. It’s the past perfect continuous(progressive) tense. It shows that an action that started in the past continued up until another time in the past. So the "playing" stopped when the bell rang. *********** We were playing when the bell rang. It’s the past continuous verb tense. It shows action at a particular moment in the past. The action started before that moment but has not finished at that moment. So the "playing" may have continued when the bell rang.