If maybe soap residue from when is was cleaned last. Possibly used too much, wrong kind of soap, or didn't rinse it well, hard to say. For a quick fix try some carpet powder or even baking soda (can even add some essential oils for fragrance) ,maybe even baby/talcum powder. Sprinkle it on really heavily, let it sit for and hour or so and vacuum. This may be just a temporary fix, can't hurt.

You can rent carpet cleaner inexpensively at some home improvement centers. Don't buy one. The ones people buy for home use are just never any good, except for spot cleaning. The ones you can rent are much more powerful and have better suction. I always clean with "soap" first then go over with plain water to rinse. Only use the recommended cleaning solution, I usually use 1/2 the recommended strength or so.

I am a landlord. If a new tenant came to me and told me about this condition, I would offer to pay for the rental of a machine. Also, after using the cleaning solution, go over it again using vinegar where you would normally put the cleaner.

If I was a new tenant and I found this condition in my apartment, I would expect the landlord to come over with that rented machine and do the cleanup himself. Note: I was a landlord for many years.