there is strange sound coming from my laptop fan

@muhammadha21836 could be an issue with the fan motor or with debris, that has collected on the fan blades or the air inlet. Time to disassemble your laptop. Use the Dell Inspiron 15 5515 Service Manual to work on it. Page 35 will show you how to work on the fan. Clean the fan as well as all the air inlets etc. I would even consider changing it since it is a vital part of your laptop. Spending the money to ensure the laptop will continue to work for a long time, seems to me as a good investment. Replacement fans are readily available at many online stores.