Musculoskeletal strain or spasm: This is often the most common cause of intermittent flank pain without other significant findings like kidney stones, hydronephrosis, or hematuria. The pain can be related to muscle strain from heavy lifting, bending, or even poor posture.

Pyelonephritis or urinary tract infection (UTI): Although hematuria is not present, some UTIs can cause flank pain without visible blood in the urine. The pain is usually accompanied by other symptoms like dysuria, frequency, or urgency.
Nephropathy due to analgesic abuse: Long-term use of certain analgesics can lead to kidney damage, causing intermittent flank pain.
Renal cysts: Simple renal cysts are common and can cause intermittent flank pain, especially if they become large enough to press on surrounding structures.

Abdominal aortic aneurysm: Although less common, an abdominal aortic aneurysm can cause intermittent flank pain and is a medical emergency if it ruptures. The absence of other symptoms does not rule out this condition.
Pulmonary embolism: In rare cases, a pulmonary embolism can refer pain to the flank area, especially if the embolism is large and affects the lower parts of the lung.
Retroperitoneal hemorrhage or tumor: These conditions can cause flank pain and are critical to diagnose early due to their potential for serious complications.

Renal artery stenosis: This condition involves narrowing of the arteries that supply blood to the kidneys and can cause intermittent flank pain, usually accompanied by hypertension.
Papillary necrosis: This is a rare condition where part of the kidney tissue dies due to lack of blood supply, often associated with analgesic abuse, diabetes, or sickle cell disease.
Amyloidosis: A condition where abnormal proteins (amyloid) accumulate in various organs, including the kidneys, potentially causing flank pain among other symptoms.