Compared to traditional cryptography methods, physical layer secret key generation (PLSKG) can be more efficient and well-suited for Internet-of-Things (IoT) owing to its lightweight nature and scalability. In PLSKG, schemes utilizing local random generators are employed to achieve a high key generation rate. In this study, it is proposed a high-rate PLSKG in the presence of an untrusted relay. This untrusted relay assists the PLSKG process but cannot determine the secret key. We select channel probe signals from PSK signals and adopt a multi-bit quantizer at the receiver to enhance practicality. Additionally, we utilize quantization with guard bands (GB) to decrease the key error rate. We calculate the performance and security of the proposed PLSKG scheme under these conditions. Our results indicate that the relay, receiving superimposed signals from Alice and Bob, cannot ascertain the secret key. Finally, we compare the proposed PLSKG with a direct scenario where the relay is omitted during key generation.