Silence added at start of rendered audio

Hello folks, I wonder if anyone could shed any light on what's happening here, and how to resolve it? I'm rendering an audio track from Reaper which will replace the original audio track in an mp4 video. No changes have been made to the length of the video or audio, so it should fit exactly in place. But the WAV file rendered from Reaper has some "silence" added to the start of it, around 2645 samples. If you bring the WAV file back into Reaper it isn't visible, but when you open it in Soundforge it is! When the WAV is converted to AAC (mp4 container) and imported back into Reaper, the "silence" is at the start. Have a look at the attached screen capture for clarification. This has happened to me before when exporting stems for a studio using another DAW. Fortunately I had exported a click with the stems, so it was possible to resync them, but it would be good to know how to prevent this in the first place. My suspicion is something to do with PDC... Any help much appreciated, Cheers, Jennifer

I've looked a bit further this morning and found that the AAC codecs add a bit of silence at the start of a file, and apparently it's a feature, not a bug. So nothing to do with Reaper there, but looks like I'll have to use a different format for the rendering the audio to go with the video. I'll have a look at the stems issue later. Cheers, Jennifer

Thanks for confirming this. I went back to the project with the stems problem, and can confirm that I rendered the stems as mp3s - they were just guide tracks for the recording in the other studio. Rendering the stems as WAV files fixes it. FLAC and OGG files are also ok. Cheers, Jennifer