Yes, I'd agree with Wandgrudd's assessment - the box labelling and logo does look like it might be quite an early factory-decoder installation - can you remind us, what's the build date of the Spreewald (the first and last digits on the gold sticker)?
If that is the case, then - as mentioned above - it is probably a 55020 decoder (made by Lenz around the time that the first MTS1 system was introduced).
It should run fine if you can ignore the buzzing - it will lack the more modern features like the back-EMF facility, which helps to keep the loco running at a constant speed on curves and gradients rather than slowing (or accelerating downhill), but otherwise it is still a perfectly functional decoder.
Should you find you had the budget and the desire to upgrade it, you could swap the 55020 out for a 55021 - or better (and cheaper!) either a Massoth L or one of Modell-Land's mXion L models, any of those options would be a simple plug-in refit.
Jon.