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These questions may seem obvious but you didn't give us much to go on. Thank you for including your device and Android version. That can be helpful in some cases.
We'll have to assume you don't know where the alarm is coming from and that there are no pop-ups to dismiss or snooze the alarm. My first question would be is this correct?
Aside from the regularity what makes you think this is an alarm? It doesn't exhibit any of the classic signs of being an alarm with no way to dismiss or snooze it and two hours that is rather extended.
How long ago did this start? Since you say it is only happening on Wednesdays you may have to think back farther than when you noticed it start, but did you install any apps around this time, or make any system changes? Perhaps got bored one evening and started digging around through the settings or just installed an app to try for a night or a week? Anything that you can tie to the time of this beginning or just before?
Are you able to identify the ringtone being played? Is it a stock ringtone or one that you use? By changing your notification sound or ringtone in your settings are you able to cause this tone to also change? I realize this may be difficult to test since you only get the sound once a week but it can help investigate the source.
Without answers we're at a standstill for what we can really suggest that you try but here are a couple of things you can try.
Not knowing where the alarm is coming from it could be possible that it is coming from your clock/alarm app, because we just don't know right? You could try clearing the cache for that app and force stopping the app. you may not be able to force stop it if it is a system app but you should be able to clear the cache and maybe even the data. Being a system app you won't lose anything except maybe your alarms that you actually have set. Easy peasy to reset those.
If this is a notification of some kind vs an alarm, and maybe even if it is also an alarm, you could try taking away permissions which can help you identify the app and/or just stop the annoyance. Open your Settings>Applications>Permission manager>Music and Audio, remove permission from all of the apps that you can here>go back to the permission list>Notifications, remove all of the apps that you can from this list. Now obviously you don't have to remove your messaging app, email app or other apps that you absolutely trust but it will be better to remove as many as you can without totally shutting down your phone, if you know what I mean. None of this is permanent. As these apps need these permissions you can grant the permission again like it was the first time you used the app. This could be how you identify the problem app. If you've gone a week without the alarm and suddenly your photo editing app asks for permission to send you notifications and you say yes and the alarm comes back it could mean that your photo editing app is the issue. Obviously these are just examples but again I'm sure you understand. While you're in there taking permissions away you can also be looking at what has these permissions and do a bit of bias judging. Does your photo editing app really need permission to play music...?
Let us know what you find and we'll offer more advice.