A persistent unpleasant smell in the hair and on the scalp, described as sour or cheesy, can be quite frustrating. This issue can stem from several potential causes:
Causes

Seborrheic Dermatitis: An overgrowth of yeast on the scalp can lead to flaky skin and odors.
Poor Hygiene: Infrequent washing can allow oils and dirt to build up.
Product Build-Up: Excess styling products can cause residue on the scalp and hair.
Scalp Infections: Bacterial or fungal infections can produce unpleasant smells.
Diet: Certain foods can affect body odors, including those that may transfer to the scalp.
Solutions

Proper Washing: Use a gentle, clarifying shampoo to remove build-up. Consider washing your hair more frequently if you currently wash it less.
Exfoliating the Scalp: Products specifically designed for scalp exfoliation can help remove dead skin and residue.
Medicated Shampoos: If the problem persists, consider using shampoos containing ketoconazole or zinc pyrithione, known for treating dandruff and yeast.
Dietary Changes: Evaluate your diet; reducing dairy might help if that’s a contributing factor.
Consult a Dermatologist: If the smell continues despite these measures, it would be wise to consult a healthcare provider or dermatologist for a professional assessment.
Maintaining a clean scalp and good hygiene is crucial, but seeking medical advice could provide targeted solutions for your specific situation.