^^
Im had tat ups, with tat same symptoms and after sum days (i dnt wanna scare u but..) tst UPS died on me and then i got myself an APC UPS (not the one in my siggy) but APC BACK UPS ES 500 (wich is also a stepped sine wave UPS) for Rs. 2.2K...
Nevertheless, i got tat Numeric UPS repaired, the service guy changed the battery inside it, but even then i hv never used this UPS (with the fear of exploding or behaving it in an abnornal way)..
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But, the rattling noise (if it has started coming since few days, off late)... it cud be tat there may be heavy power fluctuations or tat the cord wich goes back of the UPS is a bit loose...