Solid animated gradient, no external images.
Free, built into your browser — no account needed. This only plays during Preview so you can check pacing; it can't be baked into the exported video file (browsers don't expose TTS as a recordable audio stream). For narration in the exported video, generate a free MP3 at a site like ttsmp3.com and upload it below as your audio track instead.
Pick a track you've downloaded — YouTube Audio Library / Pixabay Music for free music, or ttsmp3.com for a free narration MP3. It plays under the export and gets trimmed to the video's length; nothing is uploaded anywhere.
Preview loops in the canvas. Export records the real canvas frame-by-frame into a downloadable video — no server, no account, nothing sent anywhere. It tries MP4 first and falls back to WebM if your browser can't encode MP4 — since this is just for you, whichever one your browser gives you should be fine to use directly, or you can drop the WebM into VLC/ffmpeg once to convert it if you specifically need an .mp4 file.