Microservice Communication: Sync versus Async

Hey everyone, I've been thinking a lot about how microservices talk to each other lately. We're doing more and more calls between services that really feel like they could be done in the background, but honestly, the quick feedback from synchronous REST calls is just so easy. What do you all usually do to decide between synchronous REST and something like Kafka or RabbitMQ for messages between your microservices? Are there any specific signs or rules of thumb you follow?