Currently renting but looking to buy in the spring or summer. With rates likely to keep rising, I would like to lock in a rate sooner rather than later. What can I do if I don't have a specific property in mind or even an exact timeframe? Obviously, I have a pretty good idea of overall value and loan amount that I am going to be looking at.

You can lock in a TBD property. Up to 90 days is free in that there is no upfront money. (but 90 day rate lock is a slightly higher rate then say a 30 day rate lock) After 90 days it usually costs up front money to do so.