In Exodus 28:33 and 39:25 we read that the high priest wore bells on his garments. I read somewhere that this was to let the priests standing outside know if he was still alive. If the bells stopped tinkling, he was in trouble. Then they would pull him out of the Holy of Holies by a rope. Is this true?

No, I do not think that is right. I have heard this idea, but its biblical basis is shaky. Yes, the bells were there, and the Old Testament does speak about them in the context of the danger of being in the presence of the Holy. "This robe is to be worn by Aaron when he ministers in worship, so that its sound can be heard when he enters and leaves the Holy Space in the presence of Yahweh, so that he will not die" (28:35). But the precise function of the bells is difficult to discern. (Did they alert God? Did they tell those standing outside that the High Priest was still moving? Do they have a deeper symbolism?) The matter is addressed well at

By the way, in later Judaism (by the first century AD, at least), there had arisen the view that the bells stood for thunder.  Although I find this surprising, I will give you the reference: "When [the high priest] officiated, he had on... a blue garment, round, without seam, with fringework, and reaching to the feet. There were also golden bells that hung upon the fringes, and pomegranates intermixed among them. The bells signified thunder, and the pomegranates lightning" (Josephus, War 5.5.7).

Finally, there is no mention of a rope. (Yet it is an interesting idea, and "preaches well.") We need to check our facts.