JPEG compression is widely used to store digital images and extensive studies analysed its impact on the image quality; in particular the quantization noise and artefacts created by JPEG. Nevertheless, there is little work on the impact of JPEG compression on the noise already present in the image. In this paper, we propose a model predicting how the noise power is affected by JPEG compression. This allows for a better understanding the noise traces on the image, which is crucial for image forensic analysis and image restoration. An interactive demo for this article is available at https://ipolcore.ipol.im/demo/clientApp/demo.html?id=77777000136}{https://ipolcore.ipol.im/demo/clientApp/demo.html?id=77777000136.